The short and common answer: Yes, they're really good - at least as good as WotC, and a lot of folks consider their products of overall higher quality. At the very least, they're comparable.
A bit more, with different areas:
SPLATS: Tome of Beasts I and 2, Creature Codex 1 and 2, Midgard Heroes Handbook, Deep Magic. I don't have the last one, but they're all excellent - especially the four monster books. I especially like the pocket sized, which are both much cheaper and handier to have at the table. Highly recommended. The Heroes book has lots of alternate classes and such, and is really good.
SETTING: Midgard Worldbook, Zobeck Gazetteer. In my opinion, the best well-known and in-print D&D setting. It is kitchen sink but closer to "magical Europe" than, say, Golarion or the Forgotten Realms. It has some really creative areas, and is just well done, top to bottom, with a more cohesive feel than other kitchen sink settings. Zobeck is the classic starting town, very Germanic in feel.
CAMPAIGNS/STORY ARCS: Tales from the Old Margreve, Empire of the Ghouls, Courts of the Shadow Fey. WotC-style mega-adventures. I only have Old Margreve, but it is really great sandboxy forest setting. Highly recommended. The other two look good, with Empire being their most recent offering.
ADVENTURES: Lairs books, Midgard Sagas, etc. I haven't investigated these too much, but the Lairs books seem really good and are tied in with the big story arcs.
OTHER: They have a bunch of other stuff, including Lair-specific player's guides, pawns, etc. I haven't really investigated this stuff. The Design series is excellent.