as i work at a paintball store, i've shot just about everything there is out there, and nothing really compares to the Nova.
Nova 700 w/ Psychoballistics Pro Style drop 18" custom machined barrel, 3k 47ci. steel HPA tank, Ricochet 2K; VM-68 w/ 12" stainless & 13" Armson barrels, SkanLine 45° ASA & Xchamber kit, bottomline kit, Starfire bolt, 2 20 oz and 1 9oz tank.
this thing is wicked out of the box, but i'd recommend an agitated hopper to keep up with higher rates of fire, and perhaps an aftermarket reg (max-flo?) to solve any shoot-down problems you may have with the stock reg. replace the euro-grip with a 45 grip immediately...it's ugly and you can't put a drop on it. a drop-forward would also be good, as it balances the gun far better.
-silent. this thing makes 'Cockers sound like prolites...and that's when you're dry firing it. with paint, your hopper will be louder than your shots.
-fast. 12bps no problem at all...14 if you fan the trigger. with no sear and no mechanical trigger interface, you can seriously rip on this gun.
-accurate. i've never seen another marker that could produce a 7 inch shot pattern at nearly 80 yrds. combined with its silence and quick trigger, this would make an excellent "sniper" or back player weapon.
-built-in expansion chamber. the Nova features small aluminum beads in the rearward portion of the body. co2 enters over them, and their massive surface area converts any liquid to gas. this gun will never ever ever freeze up on co2.
-insanely low pressure. watch people's jaws drop when you tell them your gun runs on only 95 psi.
-ultra consistent. while some may argue that this is the same as "accurate" it's not. accurate is shooting where you aim. consistent firing the same every time you pull the trigger. the fact that the gun's firing system is based on pressure and volume released by a spool valve (as opposed to the air released by a hammer hitting a cup seal) means that every shot is at the same pressure and same volume which means it's ultra accurate.
-weird-ass recoil. with 12" of barrel hopping back and forth, the gun jumps around a bit oddly in your hands. it's more of a nuisance than a serious problem...it recoils the same every time so accuracy and consistency aren't affected.
-right-hand feed. gives you problems when peeking out of the right side of bunkers.
-euro-grip. get a 45 grip...please, for the love of god...you can't even put a drop on that crappy stock grip.
-foam barrel cover. "it looks like a d i l d o" some kid said that about my gun when i first got it. it made me sad, but i had to agree. take the thing off, or get a new barrel.
-absolutely impossible to find parts. airstar went poof, then section 8 took over and went poof. you can find nothing for this gun. barrels can be had...from time to time...on eBay...from some guy making them himself in California. i picked up an 18" one from him, and it looks great and shoots better....but it took me a month to find it.
this marker rocks. if you want a superbly accurate gun that's utterly silent and will turn heads wherever you go, pick this up. if you want something you can trick out....maybe not this one then. there's not much out as far as upgrades, but then again, you don't really need much to get this thing in top form.