Definately a new barrel (stock barrel isn't terrible, but it is LOUD)
Response trigger if you buy one without it already installed
Expansion Chamber / Regulator
If you have the money (which I don't) the low pressure kit so you can use compressed air to it's full potential
Easy to Upgrade
Easy to Clean/Maintain
LOTS of available upgrades
Consumes LOTS of CO2
prone to gun hits
First of all, I'd like to say the this is a fantastic gun for rec ball, woodsball, and scenario games. I've seen alot of comments on various threads that are along the lines of "the A-5 sucks....get a <insert $1000 dollar electronic marker> instead." If you're interested in rec play and woodsball, you should probably ignore comments like this. If you're in to speedball, you're in the wrong marker category to begin with. There is a reason that pbreview breaks up markers into high-end/low-end and electronic/mechanical. I hate to be cliche but it's apples to oranges people....
That being said, the A-5 is in my opinion the best gun on the market for the committed woodsballer. Here's why:
PRICE: It may be more than a 98 Custom, but is still comparably less to other scenario guns (ex: Ariakon's newer SIM markers). I've found that the quality of this gun is well worth the price tag.
UPGRADE-ABILITY: There are TONS of mods out there for the A-5. Many of which include milsim body kits and tactical rails which are great if you're hard core into scenario games. Personally, I'd rather not lug around all the scopes and lasers and such on my marker, but to each his own. I've also found that installation is very easy. I'm not very mechanically inclined, and i was able to install an expansion chamber and upgrade bolt into mine without much trouble, something i would not have tried with my 98 (vertical adapters and power-tubes and what-not).
DURABILITY / MAINTENANCE: Tippmanns are pretty much no-hassle guns. If you oil them up before you play and clean the internals every once in awhile, they will last you almost indefinitely. The A-5 is exceptionally easy to clean because of the "field-stripping" design (BTW, they advertise that it can be field stripped and reassembled in 30 seconds or so, but I suspect that's after 15 years of doing nothing else with your time...). All you have to do is pop out a few bolts and you have access to all of the internals and the trigger parts.
CYCLONE FEED: This is one of my favorite things about the A-5. The stock A-5 comes equipped with a pneumatic hopper that feeds paint with every cycle of the gun. You literally can't out shoot this hopper. I honestly don't know why they haven't applied this same idea to speedball. It's pure genius! It does use a bit extra juice though, which brings me to the next portion of the review:
AIR CONSUMPTION: The A-5 is a definite gas-hog. This is of course, due to the gas fed hopper. All in all it is fairly efficient, but when you get to the end of your tank, the hopper starts to skip. I've found this happens in about the last 10-12 shots from your tank. If you know your hopper is full, but your not shooting any paint, then you're about out of gas hos...Tippmann has compensated for this with a manual feed button. you get one shot per click. This is slightly annoying, but if you're that out of air, then you're close to being out of the game anyway. I can live with pump action for 12 shots...
GUN HITS: The stock hopper for the A-5 has a large flat area in the front of it. It's basically a bull's-eye. Absolutely no hope for a bounce on this puppy. If you're out from a gun hit with an A-5, then chances are it was the hopper that got splattered. For this reason I recommend getting a ricochet hopper. It's much more low-profile and angular so it's less of a risk for a hopper hit.
other than these few things, I haven't found anything else to complain about
If you're into rec ball, woodsball, and/or scenario games then I strongly suggest you buy an A-5. It is a great gun that is easily affordable, upgradeable, and maintainable. At times the gas-consumption can get a little annoying, but nothing is perfect. I give this gun a 9 out of 10