Played Ultimate Doom? Played Doom 2? Played Final Doom? Still want more? This is what you're looking for.
Anyway as for the levels, kind of like TNT, some are good, others are meh. I did enjoy most of them though, and if compared to Doom 2 they have a similar atmosphere. Among my favourite levels are Black Tower and Sea of Blood.
There is not much to say. These are just new levels that are especially fun in multiplayer. There are no new graphics except for a few new textures. Overall it's just what you would want to get if you played Doom 1/2 and Final Doom, and still want more.