DM6 owner here, have owned the same one for about 4 years. My ownership experiences and stuff i've heard:
Its really, really reliable. The only time my gun has stopped working due to something not stupid (ie batteries) is when I ****ed around trying to take apart the reg without the right tools (You need a threaded stick, M4 thread I think to take apart dye regs) and broke the reg.
Otherwise I take it apart once every 6 months for maintenance. Take apart is relatively simple, as is the maintenance. Generous amounts of dow33 lube on the main bolt o-rings, and then a normal dab on the rest on the bolt, lpr and hpr.
Older boards will not be tournament legal unless they have been updated by Dye, but if you get in contact with them they'll send you a brand new legal board for a tenner.
Changes between the DM9, 10 and 11 are extremely minimal as far as internals go. DM9 on the inside is pretty much the same gun as a 10 or 11.
2nding the Seventh Element board with the rotor board combo, it has some really cool features
And probably my favourite part, Dye support is AWESOME. I was having troubles at big game last year with my DM6, I went into the dye tent, 4 guys sitting at a table ready to help. Gave my gun to them, they immediately started working on it (turns out it was batteries again lol)
They replaced the battery for me, and then gave it a tear down and a clean/lube just for good measure, on a 5 year old gun at no charge.
So to sum it up. I still shoot my DM6 because its a lovely gun to shoot, I went through 5 guns in a year before finding the DM, and have stuck with it because of what I found. Unbeatable reliability and keeps up at 10bps like any gun can these days. If i had to upgrade for the "new and shiny" factor, I'd take a DM9 as they represent some good value for money.