I've got some experience with it, although not running it "straight up". It's currently being used in a game I'm running for my wife. The world is basically all islands floating in the air, and she's playing an Airship pirate.
Uresia is being used mostly as is to fill in the "gaps" of the world. I've spread the distance out some and dropped a couple of things here and there so that it works for the particular game I'm running, but it's still mostly Uresia.
As far as the rules are concerned? You're kinda stuck. Most of the people online that even know about BESMd20 don't like it. It sits in this odd little place. It tweaks the d20 rules in some good ways, as well as some bad ways. In the text of the BESMd20 book, it comes right out and says that it doesn't think 3E is very balanced. That pissed a lot of people off right there.
The problem is that while it's using the d20 rules and mentions that default d20 isn't really "balanced", BESMd20 basically relies on the group (GM and players) to ensure a "balanced" game itself.
What a lot of people miss is that BESMd20 is using a different yardstick when it talks about "balance", and it's one that most of the d20 fans don't agree with, don't like, and/or don't see.
As for using the BESM 2E rules? Again, you've got the potential issue of "balance". If somebody wants to, it's not that difficult to make an _extremely_ competent character. The philosophy behind BESM (d20 and up through 2nd Ed) is that even though you've got points assigned to something, it's up to the GM and the players to decide if something is "broken" or not.
In other words, just because it's a point buy system, don't assume that 2 one hundred point characters are going to be equal in all respects. There's this false expectation and demand that if points are being used, then _clearly_ it should be balanced. Of course there's all kinds of variance in terms of power between the different classes you'll find in D&D, but when you're using a point based system to free form create a character, it's much easier to see these imbalances.
BESM is a "dead" game, using the online gamer yardstick. Regardless of the version you plan on using. The company (Guardians of Order) is out of business, White Wolf got the rights to most of the GoO properties but hasn't done anything with them. There was a single print run of 3rd Ed BESM, but it's pretty difficult to get a hold of a print copy. There's no support material being produced for it, and it doesn't seem likely that there's going to be anything else made for it either.
In other words, if you like to buy supplements, look elsewhere. All that's out there is probably going to be all you're ever going to get.
3rd Ed BESM is also significantly heavier in the rules department. 2nd Ed already added a greater level of complexity than many BESM fans wanted, and 3rd Ed took it even further.
As for the overall tone of the setting?
If you're into it, it's pretty fun. It's no more difficult to maintain the tone of Uresia than it is any other setting. The trick is whether the tone resonates for you in the first place. If you've seen Slayers (I've just started watching it) or Rune Soldier, you've got the basic tone. I think. There's an undercurrent of humor to it, some slight naivete, and virtually no sitting around and angsting about whether it's "realistic" or not. Anime that seems to have inspired Uresia has its own internal set of assumptions about what's important and not in terms of how the world works.
I've got the BESM version of Uresia, I can't remember if I've got the d20 version of it or not.
Sidenote: S. John Ross (the creator of Uresia) is working on a systemless version of it right now. No expected date on when he'll be done with it, but it's coming along.
You can find his website here:
http://www.io.com/~sjohn/uresia.htm
He's got all kinds of nifty bits at his site and there's a link to the yahoo group for Uresia as well.
Is any of that helpful?
