We used SafeStyle and we've had to have them out twice a year, every year, for 3 years to fix the windows/doors. The main issue is that the windows whistle because the fitters used (and amazingly continue to use!) sealant that shrinks over time. This allows an air gap which whistles when windy. Its not a lot of fun trying to sleep on a windy night when the windows are howling away.
Also, I've just rung them to come out (for the third time) to adjust our back door. It "drops" apparently which means that it is a bugger to lock.
The customer service is OK (they do come out) but its annoying to have to keep getting them to solve the same two problems.
My 2p.
C.