Here's the thing, at the price these things are going for, and the performance they are CAPABLE of, they are very appealing. As I mentioned on another forum, for $275 (+/-), if you have issues and you have to send the pistol back for service, it's a lot less aggravating than having to do the same thing with a firearm that costs three or four times that price (which I have, unfortunately, had to do). Also, if you carry it a lot and it gets dinged, scratched, etc, at least it's not your $3000 custom 1911. Lastly, if you are involved in a defensive encounter, and the pistol is seized as evidence, you don't have an expensive pistol languishing in a evidence locker somewhere for several years until the legal system runs its course (yes, it may still have to stay there until the civil proceedings are all done, not just a criminal investigation).
That said, the R51 is an easy to carry, good shooting pistol chambered for an effective cartridge. It's only downside is/are the nagging reliability issues. If Remington had taken the time to iron all that out and delivered a rock solid reliable pistol out of the gates, the R51 would today probably be enjoying sales and reviews similar to the S&W Shield. I say, if you're interested in the R51, take the chance and see what you think of it. If you don't like it, you're not out much. If you have problems, Remington will fix it (by the way, there Customer Service folks are quite nice to work with, and the return and repair was relatively hassle free). At best, you end up with a nice little pistol with a unique appearance and operating system that will likely garner more than a few comments from other shooters. At worst, you trade it in for loss (which given the relatively low price-point, won't be all that much), and get something more to your liking. Probably the biggest negative to owning an R51 are the internet comments from those who say: "I wouldn't own that _______! It's an unreliable piece of _____!". Many, if not most, of the people who say that have probably never even handled, let alone fired an R51. If they had, their opinion might just change.
HRF
When the RM380 can be had cheaper, is much smaller, and has much reduced reliability issues, I think that's a better route... When first announced, I definitely wanted the R51, but giving it closer review vs other single stack 9mms, I just didn't see the overall benefit, especially in light of reliability issues... YMMV.
On size relative to RM380, this is what I found based on #s at this link:
http://www.shootingdaily.com/news/remington-has-listened-here-is-the-answer/R51 is 14% wider, 26.8% longer (including 1/2" longer barrel), 20% taller (but +1 on mag capacity), and 85% heavier vs the RM380.
That 1/2" barrel length difference is roughly a 50 fps advantage; however, 9mm does provide ~40-75% more energy in similar loadings in same barrel lengths (here w/ difference more 50-80% range), which matters most not so much as energy persay w/ handguns as it does with having the ability to push fully expanded HPs, etc, defensive bullets to adequate penetration depths... Shot placement is paramount w/ handguns.
But since I live where it's T shirt weather 9+ months of the year, that energy difference is less of a concern to me than if I lived where it was coat weather 1/2 or more of the year...
Notably -- with Cabela's sale, Remington rebates, buying cabela's giftcards at less than actual cash value (~10-20% discounts are regularly available on the interwebs), and no FFL fee since picking up in store, I snagged the version w/o the laser for sub-$150 and the versions w/ the lasers for sub-$250 -- accounting for the retail cost of the lasers, the price I paid for the 3 RM380s I purchased was less than $100 per gun IIRC -- would have been 2017 pre-holiday rebate program. Seriously looked at the R51 under the same program and similar costs to the RM380, and that's when I made the call to skip investing in that weapon system.
Granted, I also work w/ a live, work, and drive w/ double stack compact 9mms close at hand or on hip, and can also quickly access carbines if the need requires, so concerns about the .380's capabilities are lessened due to often immediate alternatives -- but to the point, I think the Remington RM380 is a superb concealed carry option during warmer months where you and any threats will be wearing lightweight clothing. YMMV.
Please keep us posted on performance. I never say never on owning any type of gun, but the R51 is currently on my backburner for the forseeable future. Perhaps user feedback will surprise and make it a have to have.