I love 5 level PrC's. They are my bread and butter. I haven't made a single 10 level class, but I've proposed 3 5 level buggers in my curent game. I love them to death because they are all I need- a short dip into specialization. You shouldn't have to spend half your progression on something to become a master, and you should be able to master several things in the life of your character.
I've made a Stealth class that gives up sneak attack and even an average attack progression to be able to sprint stealthily and dissapear from plain view even without magic.
I've made a Spellbreaker class specializing in dispelling and counterspelling, while still being able to cast offensively.
I've made a Force warrior who gets a good attack progression, a couple of caster levels, and combat abilities revolving around telekinesis and force effects.
None of these ideas needs 10 levels to carry out, (and with the Force warrior I actually had some trouble bringing it to 5!) You don't need to throw in bonus feats, or blank levels, and everythign fits just right. Plus, if you plan it right, you get 3 completed PrC's by 20, and that's just cool.
- Kemrain who has Seen the Glorious Light.