I've long had a soft spot for this OVA. Particularly the first two episodes, where the supernatural horror is consistently involving; after that it seems to gradually lose its way. It was based on a novel that was originally adapted as the film Tokyo: The Last Megalopolis, released in 1988. It's pretty different.
As films go, it's pretty long at over two hours but they still have to rush through a lot. The OVA makes its own cuts to the story, particularly the fact Yukari is descended from Masakado, which is addressed right at the start of the film (and has nothing at all to do with the OVA, I think for the better).
To say the film rushes into things would be an understatement, so maybe if you're thinking of watching it, watch the first 2 episodes of the OVA beforehand as they condense the whole of the first into a very small space of time, with all characters being active fairly early on. You barely get to know any of them though, and it's not really explained what anyone's doing. There's then a jump in time after the second OVA anyway so it's not like you'd break the flow.
Don't quote me on this but I think most of the first 3 OVAs take place in about the first hour of the film. [What seem like] the lead characters of the first 3 episodes play almost no part in the film (Narutaki and Yukari have no contact, which is weird since their relationship is almost the basis of the OVA). And Yoichiro couldn't be any more different.
The incest is barely touched upon and only briefly alluded to (unfortunate, given it seems to be fairly important, and is played fairly horrifically in the anime). It's really weird at the end the film how they're suddenly, vaguely as all hell "Yukiko is the daughter of Yoichiro! And they all have super Masakado powers!" WAIT, WHAT? *acts like incest ain't remotely worth mentioning beyond that*
I much preferred the Gakutensoku thread climaxing with the rest of the film; the whole moon strand from the 4th OVA didn't really add much apart from that awesome image of the chain-pulling Katos, and Keiko's ending was anticlimactic as it was lacking build.
I'd say the OVA's easily better for the supernatural battles, which are pretty fantastic (different, but surprisingly good for the time), which in the film are relatively hurried over but it does have some neat effect work (for '88). There's little resemblance to any of the set pieces in the anime so it's not like you're watching the same thing twice.
Creature designs were actually by HR Giger (!), though there's not much to really get excited about if you're a fan.
Also, the movie's music is EXCELLENT
However, neither film nor OVA properly explain WHY KATO'S DOING ANY OF THIS ~ It doesn't feel like his backstory ever reaches any kinda of closure.
IIRC the old English dub even made out Kato was attacking Tokyo to get at Dalian (due to dying there in a war), when the opposite is actually true. One thing I particularly miss in the OVA is a cool scene in the film where Kato has a Chinese cult chanting and stuff in Dalian in some subterranean enclosure that looks like a pool of lava (they're trying to summon the underground energies to get at Tokyo).