For my first playthrough I didn't do any summoning, even NPCs. You're right, it's much more satisfying killing a boss by yourself. After that I did a bit of New Game+ and then took on EpicNameBro's (he's a guy on YouTube with some great Dark Souls videos) Class Challenge. The rules are: no equipment allowed other than starting gear (except rings), no healing during boss fights, no leveling up, no covenants, no co-op, and a few other things. I got to Ornstein & Smough before stopping. I guess you can call that "twinking" kind of, except I didn't do online. After that I finished New Game+ and the trophies. Now I'm getting into co-op finally and it's been a ton of fun. I made a Solaire character build (Praise the Sun!)
Also, I love your sig dude.
I guess I'll avoid co-op until NG+. I just killed the Moonlight Butterfly without co-op or NPCs, and it was sooooo worth the ten times he (or she?) destroyed me. I don't have any casting spells at all and that fight was a ***** to get through with just melee.