Illusion is lots of fun, I agree. Lots of laughs for me if it's done right. My pure mage character http://i936.photobucket.com/albums/ad208/xenaclone/Image0085.jpg has struggled to incorporate as much Illusion as she can. She does participate in all the other schools, though. I have yet to make a pure illusionist.
Beth's main schools (most-used to least-used) go like: Alteration, Destruction, Illusion and Conjuration are about equal, and Restoration is the least-used.
I'm waiting to get to the point where Beth finally gets Invisibility. Her entire game is going to change when that happens. But I've noticed Illusion levels up really slowly. She's at Level 30, and still hasn't seen Invisibility offered to her.
Use courage on your follower before every combat situation, it helps to level it up quickly and it makes them quite a bit stronger. And yes, you need around ~70 illusion for invis to show up in the vendor list.
The reason I prefer illusion over conjuration is you have more control over situations: sometimes NPCS will ignore your summon and bolt straight for you. In that case I'd quickly calm the stray target, and rip into the frenzied crowd with fire or lightning at range. The only thing I really miss is Dremora Lord
