I have used both OBMM and FOMM. While OBMM is very functional for it's intended purpose, it is a bit clumsy to work with (just my opinion) FOMM however seems much more refined and a whole lot easier to work with. personally I would like to see it done.
I do have some experience with Wrye bash and also Gary Bash, but since bot Oblivion and fallout are very stable I really don't see much need for me to use them much.
OK as you all have probably heard I'm a big proponent and fan of Wrye and Gary Bash - that said there are features that make me like to use FOMM whereas I try to really never use OBMM if I can help it.
The way you can drag and drop files is a big one - you can highlight and drag more than 1 file and when you do the window scrolls with you. So no more of this drag one file to the top then move the window and drag further.
FOMM does offer file by file control over each replacer or added mesh, texture, sound, etc. So you could install a replacer pack (or several) then go through and choose from which pack what you want to overwrite as well as get detailed and more meaningful than OBMM conflict reports that tracks not only what is installed via FOMOD but also loose (although not much detail there). BAIN does not track loose files only tells if if what is installed with BAIN is overwritten. You can't control the loose files though.
FOMM also has BOSS integrated (although not sure how updated the masterlist is for it), but presents it in a different way. You get two read outs - what FOMM sees as compared to the masterlist and what it recommends. This feature could use some more work and most likely help, but it is far more than what OBMM can do.
Kaburke - I truly thought there would be more interest. guess I was wrong.
And to all you Obliviacs not playing Fallout - why not? The mods have come a long way - FWE/MMM is a great combination and the world space is huge and there are some cool quest mods out there. Bethesda really did the dlc right with it - each is an expansion of the game - not just a dungeon or set of books. Modded Fallout 3 is a great game and by far it is many many times more stable than Oblivion. I run a few 100 mods on it and really unless it is a direct and florid conflict or dirty mod that I'm trying to force into working the game rarely crashes.