I had a fst 500 on my last quad. I have mixed feelings about. It did make awesome power and torque. However, if you had it geared stock it was hard to keep the front end down and if you geared for top end it was hard to ride through the trails slow. Also it was a big money pit and I am embarassed to say how much money I invested in that quad trying to be faster than 450's. I finally gave up and bought a 450. I also had a 440 motor that same quad and I would say reliability was similar but both were well built and done right. If you do a 500 or 440 I strongly recommend a carb upgrade and 450r carbs from 04-05 work great. Also heavy duty head studs is a must. Auxuillary cooling is another must in my opinion, larger oil tank and aux cooler. I did have alot of fun upgrading my 400ex and it was alot faster than stock and I learned alot doing it but what a money pit.
2006 Honda 450r with HRC kit , HMF slip-on, pro-armor nerfs, EHS Racing airbox lid.
1999 Suzuki RM 125
2004 Yamaha YFZ450