Some background information to help "index" my experiences. I'm relatively experienced (5 years or so), my wife is my key-holder, we play off/on for 2-3 months at a clip. When "on" I'm pretty much in 24/7 so long-term wear is critical. I'm around 50, decent shape, normal size, a grower and not pierced. I've tried CB-2000, 6000, 6000S, Mature Metal, Neosteel. At current, the CB-6000 with a KSD G3 anti-pull out addition from Kept of Her has been my favorite for a balance of comfort, security, etc.
Like many chastity "geeks" I've continued to search for upgraded security without loosing comfort or getting pierced (a non-starter for my wife). The Bon4M interested me as it was like my current favorite (CB-6000) with the potential of being more secure given its metal construction. The idea that I can just snap off the CB-6000 if I get really pissed is a mental block for me. The Bon4M also seemed pretty reasonably priced for a metal devise so I didn't mind buying for experimentation purposes. I had also heard from Kept for Her that their KSD had been modified (by another user) to fit this devise - something I considered critical as all other ball trap devices without the GSD are just too easy to come out of with my anatomy. I at least have to be able to pretend I'm in for the long haul for me to get into the right mental frame of mind. The ordered devise arrived from China about 3 weeks ago (14 days from order to delivery).
IMPRESSIONS
Overall, the devise is one of the better price/value metal devises I've had or seen. As delivered, it's got some problems (see below) but overall it was very much like the CB-6000 which I like very much. My feeling is that the fit/finish is OK on the outer surfaces - not OK on the inner surfaces - especially within the cage. I didn't even attempt to wear it until I'd spent time grinding, cleaning and polishing. If you're handy and have a Dremel, I believe that this can be turned into a very good (perhaps even excellent) devise and may be close to "the one" I've been after. It is NOT ready to slap on and go out of the box in my opinion - just too rough for long term wear (or even a weekend).
So for those that are interested, here are the things I did with the devise to bring it to where it is now...
Cage: I like the design allot (as does my wife

After the heavy grinding was completed and all surfaces carefully checked for burrs (a few very uncomfortable mornings as I found I hadn't checked quite closely enough), uncomfortable edges, I smoothed and polished until it felt universally smooth and looked very shiny. Note: Access to the inner cage can be a challenge and I couldn't (or didn't) get a mirror finish inside but perhaps with enough time (many many hours or better tools) it could be done.
Anti-Pullout Addition: With the cage complete, I then fit a spare KSD anti-pull-out devise which proved pretty easy. Just drill 2 holes in the plastic to allow the permanently mounted pins on the cage through, reshape by sanding/light grinding and heat to get a smooth fit in the tube and it's ready to go. I found out too late that I should have kept it larger than the unit in my CB-6000 (which I used as a guide). The Bon4M is not oval and the tube diameter (height) is larger by as much as 5mm so I need to re-order from K4H and leave it a bit larger to create a bit more security. As it sits now, it hard to pull out in the shower but probably not impossible - reasonably easy fix.
The Ring: I use the 48mm ring on the CB6000 but hoped to tighten this up (metal, more weight) and went down to the 44 mm (4 rings come with the devise - this is the second to smallest). I prefer the rectangular Bon4M ring design to the round one (like my MM) as the rectangle (like CB6000) doesn't tend to cut into me as much. Like the cage, the interior surfaces need work. It's hinged and that need allot of smoothing, cleaning and rounding of all edges - same to the entire ring. Same approach - grind the surface smooth and round every edge you can find. Polish until mirror smooth and shiny and your good to go.
The spacing on the Bon4M is fixed at what I measured as 18mm (ring to device). This is too much for me as I'll get one testicle that slips out when I'm hot, sweaty and working (amazing the things we come to understand about ourselves). I need between 11-14mm and this took some work. Basically, I didn't want to mess with the fixed pins in the cage so elected to drill out the two female holes in the ring. This friends was a bitch - probably went through 8-10 bits to get the stainless rings drilled 3-5mm deeper. The pins are graduated and I elected to just drill hold for the larger diameter. Careful not to drill through the ring. The addition of the 3-5 mm of depth allows me to set the cage/ring distance with a spacer (which they don't use). I just used some spacers from the various CB series I had and ground one down until I got the depth where i wanted it. The devise remains rigid when assembled - just be careful not to drill the ring guide holes too big - they are what provides the rigidity.
Pin: Because the Bon4M is not designed for spacing adjustment, there is only one stainless pin (actually they provide a 2nd one as well) which is standard round - not oval like the CB6000. The pin has a hole where the lock goes (cheap lock is provided). Because I wanted the spacing adjustable, I fit a second spacer on the pin behind the lock once I the depth was where I wanted it and it keeps it rigid. Not elegant and one more fiddly piece to keep track of but it works for now. The post also extended beyond the lock for 3/8" - not what you want for stealth so I ground that down and polished it up. Overall, much better than the CB6000 in terms of security - I know I can't just break off the pins and be out if I wanted to. That said, I would say that the pin and locking mechanism remains the weakest part of a pretty elegant package. The lock is easy to pick of cut off and sticks well away from an otherwise nice package. Something to think about for the future I guess.
HOW DOES IT WEAR
I've been in the "fiddle" / "wear" / "fiddle" mode for about 10 days. My wife has informed me that I'm to hand over the key tonight so looks like I'm going to have to live with what I've done for awhile. I had to abandon the smaller ring and go to the 47mm one which is similar to the CB. Nights were impossible and it wasn't a good kind of pain. Beyond that, I really like the Bon4M after the modifications I've made. Far superior (for my anatomy and goals) than any other ball-trap devise I've owned and worn long-term.
It fits nicely, isn't terribly heavy, is easier to put on and take off than the CB6000. The hinged ring isn't ideal but with the work I did, it is proving to be very comfortable without any kind of covering over it. Pull out is about like the CB-6000 but can be improved with a revised KSD design which I'll do (they may even be considering doing one for this devise if sales take off). I like the look - especially after polishing and the fact that it's metal make a difference to me; knowing that this will not be coming off without a key or a Dremel. I would say it's slightly less concealable than the CB6000 but similar. Hygiene is better (by far) and so far, I would give it very high marks for wear-ability and security.
Sorry for the long post but it may be that some of you continue on your quest for the next perfect devise and have eyed the Bon4M as an option. I hope this review proves helpful. I'll check back in after another few weeks and let you know how things are evolving.