obvious stuff
well, you are asking in a S&W forum, so the answer to which gun is the M&P-10 of course!
That said you should read through some of the other threads on here for good and bad stuff. In general the good stuff is that it is reasonably accurate and the price is in the proper range. On the bad side it has trouble with some ammo feeding properly, CBC in particular seems to be really bad, and it has some parts that are proprietary to S&W. I'm not sure in general how standard the other manufacturers of AR-10 type rifles are either, it seems like there are more variations than are seen in AR-15 clones. Also since the M&P-10 is relatively new on the market the accessory makers are just getting out parts that are made to fit it... for instance a good quad rail handguard was just finally shipped that was listed in the S&W accessory book a year ago, there is a thread about those within the last week or so. Magazines had been hard to find and some were of poor quality in the beginning, i think that has improved now. But others are finding that some after market charging handles don't fit the proprietary S&P bolt carrier, and there have been long discussions of buffer tubes, springs, buffer weights, and spacers needed if you try to change the stock for one with a different tube.
Personally I went with removing the Mass required fixed stock by drilling out the pin and adding only the adjustable part to the existing tube which is working fine. I changed the handguard for an moe one that i didn't care much for but fit, but have now gone with the new quad rail that just came out and am much happier. A sling, scope, and bipod/folding front grip complete the setup. The one thing I would recommend is to get one without the fixed front sight on the gas block if you can. That makes setting up optics easier and you can always add flip up iron sights to a rail if you want them.
