My 900SL has run back-to-back track days sessions on several days, running for 6x40 mins solid with an oil temp of 150 degsC. I also tend to ring its neck when out on the road, but it's difficult to put a bike through the same abuse it receives on track. Like MDB says, keep an eye on the frame around the headstock and the swingarm shock mount and you'll be fine.
My bro and I also used to ride 900SSs for a few years, his spent most of its life on the back wheel and never suffered. Again, keep an eye on head bearings, but the rest should be fine.
Like all bikes, the reliability of the bike is completely dependent on you treating it right. If I take my bike on track and its 32 degsC air temp, and I wring the nuts off it for seven sessions, then I'm pretty likely to change the oil before doing so again.
Personally I wouldn't have anything else, I love the 2v 900 engine.
Cheers,
Ali