Well, you can get cigar/cigarillo-sized units (Like the eGo Twist and Spinner), or boxy-mods..
Personally, I wouldn't buy anything except Variable Voltage (VV) anymore - but I tinker with my own wicks and coils.
Past that, your biggest issue will be the atomizer-component. I stick with tank-styles like the Vivi Nova, Mini-Nova and so on, (smaller units are called Clearomizers instead of tanks).
I also prefer boxmods and think Coppers can be gorgeous.
If you want to get "wet" and switch immediately, then I'd recommend the eGo Twist or Spinner kits. From my homepage:
Here's what I would piece together if I was starting all over again, (single-sourced for convenience):
2x eGo-C "Twist" 900mah batteries (black) 1x Lanyard (better than sitting on it!) 2x eGo USB charger-cables (4.2v, 420ma) 1x USB Wallwart 1x USB Automotive Adapter 1x Vivi Nova Tank Kit (it includes 3 atty) 1x eGo Thread Protector/Stabilizer (covers eGo outside threads and provides a NICE, stabie base for tanks using the 510 adapter only) Several 15/35/60ml bottles eLiquid/eJuice - I like USA Red and Desert Ship. (chain-smokers can expect 30ml used in less than 2 weeks)
I managed to drop handrolled smokes due to the eGo, and I'm still using my original Nova. I no longer buy eGo and deal with boxes now.
Hope this helps!