I need to insert a note here that this is a very old thread - what we call a zombie thread - and someone brought it back to life. Some of the info is way out of date, it was good when the thread started but is wrong now. The general advice is still good but the hardware mentioned is old-fashioned.
Any new buyers guide now has to mention the eGo / Tornado / Leo and similar 'large-format' models, and probably as the #1 choice, as long as you don't need one that looks exactly like a cigarette - in which case a 510 or KR8 in white might be best, and as long as you know you'll need to carry a ton of backups going out the door, and as long as you realise it will set you up for a ton of hassle with people complaining you're smoking indoors. I think they are a bad choice, an eGo or similar wins out now, and with the small 650mAh battery it's still small.
The battery life of the mini format models like 510, 401, 901, and 901 is terrible (max 2 hours intermittent use), and for a day out you have to take 4 or 5 batteries and/or a PCC. Penstyles like the 801 are slightly better but I don't see anyone advising that as a first buy when you can get a Leo on the 801 system that is many times better.
Actually we don't have a good, up-to-date new buyers guide that features the large format models as the best choice - and we need one. As stated the general advice in this thread is OK but the hardware recommendations are RIP now.