Red labels are ok for rehearsal. I bought them back when i was a student - mostly because they were cheap! I once was in a studio that time and the engineer said "damn, go buy some good strings"
Since my cashflow is better i do not use red labels anymore. It feels like your fingers are glued to them in some way. They die pretty fast, too. But 8 euros a set compared with, lets say 30 euros for rotosounds or ghs - pretty obvious what you do when money is an issue.
Regarding sound: they can sound! Friends of mine still play them and sound pretty good with them.