I took a long time to adjust to 5s, but I pretty much play them all the time now. The main advantage I see with a 5 is it makes some patterns easier to play (i.e. you don't have to shift up and down the neck as you do on a 4, you just play across instead).
Some 5s have a poor low B tone, which can certainly give a bad impression to a new user.
I've tried a 6 multiple times, but it's just too much for me.
If I slapped a lot I'd probably stay on a 4, but I like the increased versatility of a 5 overall and won't go back.