I have a very early (July '04) model and have 42K+ miles on her without any mechanical problems. There are very few differences through the years so far. The oil drain bolts locations (there are 3, 2 for the sump, 1 for the tank) were changed early on. I have the earlier version, but the Triumph manual covers the later version..

The handlebar clamp has been changed at some point. There really hasn't been too many major changes.
There is supposedly an " upgrade kit" from Triumph that replaces the lower timing sprocket, timing chain tensioner, timing chain, and other bits. I'm sure someone will post what it contains. I haven't had a need for it.
Most Rockets seem to be owned by older people and are well taken care of, but like any bike, look for signs of abuse, such as how much adjustment is left in the clutch cable? does the rear tire look like it's been hot? brake rotors?, grooved? blue from heat? Maintenance records would be a HUGE plus!