For those who aren't familiar, the Remington Model 51 was a compact autoloading handgun designed and introduced in the mid-to-late 1910s, and sold primarily in the 1920s. Designed by the talented John Pederson, the gun employed a truly novel action design. It's sort of a delayed momentum/quasi-blowback operation that retains the inherent advantages of the fixed barrels typical in traditional blowback designs, but also features a positive mechanical lockup that allows it to handle usefully powerful cartridges without adopting the looks and weight of a brick (à la Hi-Point).

Overall it was a unique, well-designed handgun that was extremely well-received, but sadly never attained the success it deserved. The economic climate of the 1920s was not a favorable one, for both Remington and the US, and the relatively new automatic pistols were slow sellers in the civilian market regardless of brand. Also, the 51 was only offered in .380 ACP and .32 ACP, which was a bit of a waste of the excellent design. These low-power cartridges already worked well enough in less-refined, but less-expensive straight blowback pistols. Remington was losing money on each pistol trying unsuccessfully to grab a share of the market, and production ended in 1927 with less than 65,000 produced.
Cut to the present, where Remington recently announced that a new Model 51 is set to hit stores next month. The new 51 is a modernized, more compact take on the original Pederson design, but the unique action remains, with all of its advantages. This time, it's chambered for the much more capable 9x19mm cartridge, and is +P ready. Additional calibers are reportedly on the way, with a .40 S&W version said to be the next to hit the market.

I have to say, I'm really anticipating this new gun. It's nice to see an excellent, but stymied design get another shot at the success it deserved to have almost 100 years ago, and with gun ownership and concealed carry booming in the US, it's a great time to give it that second chance. 9x19 is a great carry caliber, and with a .40 S&W version seemingly confirmed, I'm keeping my fingers crossed for a .357 SIG model or conversion.
I like its layout, ergonomics, ambi mag release, and internal hammer. It's slightly bulkier then my little pocket PF-9 for the same capacity, but not by much. I'm not normally a fan of manual safeties on my handguns, but I think I can deal with a well-designed grip safety if it allows for a nice single-action trigger. Its aluminum and steel construction make it heavier than polymer-framed carry pistols, but the reported 20 oz. doesn't sound too bad, and the lack of polymer is sure to be a selling point to many. I have no issue with polymer guns myself, but I can speak from experience that featherlight automatics can be finicky and squirrely when you get up to 9x19 +P levels of power, making them tough to shoot well and unpleasant to shoot extensively. With the 51's low bore axis, clever action, and extra heft, it sounds much more comfortable than many of the polymer pocket rockets currently on the market.
I'll definitely be checking the new Remington 51 out when it hits stores, and I may just have to pick one up in the near future. If the thing is as comfortable as it seems and a .357 SIG version is made available, I may end up buying two.
More information:
Gunblast.com article on the new 51
The Firearm Blog article on the new 51
Information on the original 51