I've watched through the anime now, still a live action movie to check out. All stages 1-4 are all worth watching, though at the conclusion it is very open ended and makes me wish very much that there were some followup animes like stages 5 and 6, but since Stage 4 wrapped up in early 2006 there probably won't ever be any more (though Extra Stage 2 was released in 2008.) Battle stages are so-so, battles are better in context of full season episodes. Extra stages are so-so unless you really want to see some gratuitous boobies in the first one (Mako-chan). Main stages are all very good, probably the first one the best, since there's never a real end point to the story arc reached as you watch through to the 4th. Animation quality becomes much better through the series, though.
If you've watched the anime you can pick up the manga on chapter 33 to continue the story from there if you're really interested. Also, I can see from the manga that the animation style in the First stage was really just trying to stay true to the manga.
Watching Wangan Midnight now, pretty good highway speed, kinda creepy obsession/Christine theme I don't really need (what's with the Tuner from Hell? creepy laugh...), Initial D preferred so far.
Also watching some Shuto Kousoku (Freeway Speedway). Bastard bad cop a la Bad Lieutenant is funny. Kinda sad how they're not really going very fast, but what do you expect in real life. Funny that the series is banned in Japan, but not similar anime... Only watched the first one so far, pretty good.
Watching all of these has been educational, and I've gotten into Keiichi Tsuchiya's YouTube videos drifting the AE86 and setting the course record at Tsukuba in an ARTA NSX. Not bad for an old man! (Bunta, anybody?... well, if Bunta would smile all the time and open his eyes... okay maybe not Bunta)