Alien Interceptor, 2k5 Dragon Intimidator, DM6, ego 5/6/7/most pl's (not like it makes much of a difference), Angel 1, Marq 6/7/rapper (same gun internally), Dangerous Powers Fusion, as well as many many others through the years.
If you need a larger bore barel, thats it. The stock one is amazing. I've switched back to it after using a stiffi and an ultralite on it.
MAYBE a new board, but WAS is more than enough for most people
Lack of Kick
Ready to roll stock
Great company backing it
Board (personal preference)
The "unknown" factor of it (not many techs know these guns)
Whats there to say about the Alien Remains? Mine had a small problem, which was the noid dieing, but this was due to a malfunction in my tank, causing overpressurization. I sent my gun to AlienPB listing the problems, and about 4 days later, I got a call from the head tech, Andrew, detailing what was wrong and what needed to be done. I was charged a measly 75 dollars for a noid replacement. On top of that, I got an updated bolt, trigger spring, trigger pin, and harness. Andrew even double checked the gun before I sent it out to make sure it was fine. Since that day, 5 months ago, I've chopped 2 balls, and those were balls that sat in my trunk all winter. I get great efficency (hopper and 9-10 pods off of a 68/4500 fill), the speed is ungodly, even in semi, the accuracy is top notch, and it's just an all around great marker. It has a nice, solid build to it, and it's pretty easy to get comfortable with. Tear down is easy, and you can even troubleshoot the gun while dissassembled with air in, due to the Modular Ram Mounting system, creating a ram independant from the body of the gun. All in all, my experience with this marker and the company have been very very enjoyable. I have kept the gun for a year, haven't I?
For the price, It's hard to beat. You can find them for 600 online, and these easily fall in the paramiters of a high end gun. If you get a chance, anyone should shoot this neat little marker.