None, used to put tank to my shoulder sometimes if that counts.
98 Custom R/T Remote Line Carstock Exp. Chamber
Somewhat Hard to Tighten (can be fixed)
Plastic Wiggles a little
Can Get in way
Personally I love this upgrade. It makes my Tippmann look more like a military M4 and makes the gun look pretty cool overall. It's a pretty inexpensive upgrade. I purchased mine off eBay for only $20 (the reason I say somewhat inexepensive is because I'm not sure what others have paid for it). Even with a great price this stock doesn't skip out on quality. The rail (not sure what else to call it) that the stock slides on is made of some kind of metal. I'm pretty sure it's aluminum but I may be wrong. The stock itself is made of tough plastic that's definetly built to last. This thing is really easy to install. Just pop out the plug in the back and replace it with the mounting piece. Then just screw it on. Now on to the advantages of using this thing. First off, it steadies your "aim." The reason I say "aim" is that you can't really aim in paintball. You've got to eye it. All those who complain it's hard to aim are probably using the 98 Custom's built in sights. If you truly want to aim with this thing, go out and buy a red dot scope or an ironsight. The second advantage to this gun is that it's adjustable to four different positions. No matter what your size or preference, this stock will fit you.
Of course with every upgrade, there are some drawbacks. When installing this gun, the stock can be a little hard to tighten, and sometimes it can come loose. I personally fixed this buy puting tape over the threads at the end, and then screwing it on with a pliars. For those of you worried about scratching it, put something between the pliars and the ring. The next problem is that the stock (the plastic part) seems to be a little big for the aluminum rail so the plastic wiggles a little. It's not a big deal but it's something to point out. And finally, if you don't have a remote line your tank may get in the way. It is possible to use this without a remote line but its a bit annoying. Just add a remote line and all will be well. (and the moving of the tank will decrease the wieght your arms have to hold)
If you want a quality stock and don't want to spend a fortune you should buy this. It has way too many good points not to. And the bad points should not sway you away from this as they are really minor.