I've been vaping on-and-off for a couple years now.  I started with one of the Njoy 510's bought at a gas station.  Went from there to an actual 510 manual, to a Innokin Lea, and finally a few days ago to an Ego-T.  I had a lot of problems with juice leaking into the battery on the automatic ones.  The Ego is about as big around as a large fountain pen, whereas the 510's are more the size of an actual cigarette.  As for flavors, there is about anything out there you would want.  I would recommend just trying a few out. 
 
Edit : as for battery life, I would probably go with a larger battery than the 510, or make sure you get a few 510 batteries.  Nothing worse than to step out on break for a quick vape, only to find out that your battery is dead.