I'm a huge fan of Tippmann markers, personally... I bought a Spyder once after succumbing to the hype and it just felt cheap to me, especially when firing. I returned it and bought a Model 98 (back when they had gills, before it was the 98 Custom) and started tinkering away.
The '98, custom included, is a real tinkerer's dream. It's very robust, and you can do a TON of shit to them to make them perform beyond well. You can expect with practically any marker that you'll need to get a new barrel for it, out of the box - that's just how it is, because there's so much variation in paintball size and quality. There are some barrel sets that have varying sizes of internal bore, so you can match up your barrel with whatever paint you happen to be using. Tippmann also invented the flatline barrels, which are very interesting to use. They're oversized in terms of bore, and curved such that the paintball is forced to roll along the upper surface of the barrel, inducing a backspin. When the marker is held vertically, the paintball travels in a much flatter trajectory than normal, with an increased range. You can also hold the marker sideways or upside down to shoot around obstacles or induce a dramatic drop

The disadvantage here is that the paint you use needs to be especially round for obvious reasons, and the consistency of how vertical your marker is will be important for accuracy.
I've personally hacked my frame all to hell, because it's cast aluminum, and the clamshell design of how it's put together works especially well for modifications. If you end up getting one, make sure to say so and I'll post my marker/what I've done/what you can do to yours ^_^
With a good barrel and good paint, the accuracy is fantastic. One suggested mod in the beginning is a rear velocity adjuster, so you can control spring tension as well as the stock adjustment of how much gas is allowed to the barrel. There's a supercool one that's a rear cocking kit, as well as velocity adjuster. I want it, bad.
-m00t!