Currently I have 8 armies with at least 2000 points each and 4 with 500 points or less. I started buying when 5th first came out.
I don't jump from one to the next based on codex release. For example, my necron army was started way before the latest dex was published and although I do own the current dex and picked up a bunch of the new models I really haven't played them too much since that was released. However, I did buy the Tau codex when it became available on the iPad and picked up an
HQ, a couple boxes of fire warriors and a piranha to play around with. Although I don't see myself buying too many more Tau as the battlesuits just look very dated the models I have with end up as allies with something else.
Basically, I'm kind of an all around type. I like building and painting the models. I enjoy making lists and playing games; also I generally have just as much fun winning as I do losing as long as the game is interesting. I do have a few rock solid Necron and
CSM lists (triple drake and 'scythes) for those times when I just need to win. However, I absolutely love playing my
DE even though losing is usually in the cards.
I guess starting a new army for me has boiled down to liking the models and generally getting a "good" deal. I have a decent sized
DA force simply because I liked the models in Dark Vengeance and I found a limited
DV set for 40% under retail (shop was going out of business). I have a White Scars biker army because I was able to pick up 3
DA battleforces (previous one) for $45 each last year.
Given all that I know; I'd probably caution you against buying a
DE army right now. Unless you are going to ally an Eldar force; in which case I'd definitely say wait until the Eldar codex is released in a couple weeks.
DE is a very very tough army to handle. As has been stated, you either completely dominate or you are wiped off the table. There really isn't an in between. Last week I went 0-3 in 750 pt games. The first was lost because I screwed up objective placement. The second because I didn't use my wyches correctly (had them sitting on an objective when they should have been tank hunting - the first part of the game went so well I got cocky). The third was lost as soon as we rolled for mission: Kill points vs mech
IG on a table with very very little cover. The
IG had 3 tanks putting out pie plates. That said, playing
DE is probably the single most enmeshed experience you'll ever have with a table top game. Their entire fluff is geared towards getting off on pain. Well, watching your army be beaten to a pulp on a regular basis is certainly painful; to the point you probably need to be a masochist to continue using them.