I have used them before at work. We used to run them for our dial up isp service. We then got a contract with Dell and ended up having to swap over to the poweredge servers.
They are pretty relible for the most part. The only issue we had was as you said, not enough storage space, we upped our drive size, added a bit more memory (we were using the RaQ4r server model)
No real gripes, they did the job well. The only real issue I guess is the cost of the servers themselves. They tend to be pricy, but IMO they are worth it.
But not as expensive as that OC3 connection! That seems a bit extreme for an ftp service though, why not just go to a 3rd party host for that?