Hello.
I have been shooting the M9 in the service for over a decade now, and have personally owned one for quite a while, too. As for your questions...
1. Accuracy is on par with anything else out there, and anyone claiming something different will only have their personal opinion to back it up. Plenty of folks with little to no prior handgun experience in the military shoot expert with it, so I would venture to state that this is a fairly good indicator that there’s nothing inherently inaccurate about it. Andy Brown used a Beretta to end a mass shooting at Fairchild AFB, WA in the early 90s against an assailant armed with an AK-47 clone. IIRC, Amn Brown engaged with his M9 at over 70 yards.
2. I clean mine regularly, and service pistols are cleaned after every qualification - so I can't speak to reliability with regards to poor maintenance. The only malfunctions I have observed would fall directly on defective magazines (there was a bad batch of "contract" magazines a few years back). My personal pistol has not malfunctioned after several thousand rounds.
3. I haven't had to replace any parts, nor have I seen any excessive wear in my personal weapon. Shipwreck has an outstanding maintenance overview in this thread.
4. I hear a lot of people complain about the size of the grip. The stock grips are very comfortable for me, and I've never had an issue with this. If you are concerned about the ergonomics, then I strongly suggest you simply try one - you'll know if it's an issue after a couple of rounds.
5. I believe that most everything you could want in the way of aftermarket parts is widely available. The front sight on the M9 is fixed, and I'm not a tremendous fan of the "dot and post" (two dot) sights. I'd rather just have the 92 3 dot sights, to be honest - but again, this is a matter of personal preference. If you don't get a railed variant, L3/Insight makes a nice adapter. Extended mags/grips are readily available. I’ve been looking for an affordable (read: not price-gouged) factory .22 practice kit for a while… more on that below.
6. I think they are incredibly well-built. I don't believe in collecting guns, so I'm not a good person to ask about their intrinsic value. It’s a tool, and it’s been doing a tough job in an impressive manner for a long, long time.
As for my personal thoughts:
I would say that Beretta has either forgotten, or quite possibly, never really figured out individual-based customer service. Like many other "high-end" brands (and not only firearms manufactures), Beretta relies heavily on their name and the out-of-the-box quality of their products. This doesn't mean that you won't get warranty repairs from Beretta; rather, you'll find yourself frustrated that there are a lot of smoke and mirrors surrounding the availability of particular models/accessories (M9A1 Compact Inox.... .22LR conversion kit...). In the information age, buyers are going to be less and less likely to put up with any unknowns in a supplier – and I believe this is one of the big reasons you have seen drastic shifts in LEO firearm suppliers over the past decade(s). As an example, Glock suppliers have delivery/maintenance/sales trucks (a la SnapOn tools) in metro areas… Beretta has an 800 number. Dealing with the “unknowns” can get pretty tedious – but I do believe in their products.
You'll hear folks complain about the double action first shot... and virtually every aspect of the safety... Realize that these were stipulations of the military contract. And to be honest, I completely understand why. You would be shocked to know the number of negligent discharges in the military on a regular basis. I personally know of two. If 18-year-old kids are out there putting rounds into their feet, the ceiling of the range shelter, and the ground in front of them with a 92, then they would be doing it fivefold with a service-issued Glock.
I'll take the extra “buttons.”
Good luck.