Commoner: Yes - particularly for people who are ..well.. common. Joe average farmer, the bartender or serving girl and so on. One thing I do a bit differently that straight by-the-book, however. If a commoner has does something that's activly physical, like say a farmer - I'll give them d6 hd rather than the normal d4. If they do something that's less physically demanding, a tanner for instance, then they get the d4.
Expert: Yes - I actually really like the Expert class, because it lets you have the highly skilled people who can do lots of things, etc and they don't need to be Rogues. I also like the customizeable skill list. In my campaign most clergymen arn't Clerics, they're Experts - theologists, scholars, or people who just want to spread the word. One of the long time NPC associates of the party was one of these Church Experts, and the PCs just adored him to bits; sure he wasn't that good in a fight, and he couldn't cast spells, but he had more knowledge skills than you could shake a stick at, was a solid diplomat, could read people like an open book, spoke and read a ton of languages, and as they later found out, was a better practitioner at Use Magic Device than anyone of the PCs hands down.
Adept: Yes, but sparingly - I took a note from Planescape: Torment for Adepts. People that were 'sorta-spellcasters' .. midwives, gypsy fortune tellers, villiage healers, people with a magical talent that's really only in one field - those folks are Adepts. For things like Humanoid Tribal Shaman, etc - those I use PC classes for. But if there's a guy who is a magical tailor.. and magical tailoring runs in his family, and he's a darn good magical tailor. He's an adept. One of the changes I did with Adepts is that, like Experts, I made their spell list customizeable rather than just one standardized thing. So if you're a villiage healer, you don't know how to chuck lightning bolts. If you're a fortune teller, you probably can't heal. If you're a magical tailor, you probably know spells that you use to enchant your clothes with.
Warrior: No. I don't like the 'sorta fighter but not' class. At least with the other NPC classes, they have an individual niche that they fill. In my games, if there's something that calls for a Man At Arms, he's most likely to be just a Fighter, though not necessarily a high level one. As a guideline, most caravan guards, town watchmen, thugs, pirates, etc range from 1st to 5th level, and elites.. the royal guardsmen, the temple knights, the guildleader's right-hand-man, those tend to be 6th to 9th. Varies by situation.
Aristocrat: No. While I like the idea of the Aristocrat class, I don't like how they executed it. Instead of the class printed in the DMG, I use the Courtier class from the Rokugan Campaign Setting book. If a nobleman's just a limp-wristed court lackey.. he's probably just all Courtier. If he's the stout-hearted good guy to have at your back, commands respect from his soldiers kind of nobleman, he's probably got a couple levels of Fighter under his belt in addition to being a Courtier.
. The only possible use I can think of NPC classes if for those "friendly" NPCs when PCs are level 1.
