antirez (the author of Redis) says good things about MongoDB, but like him, I found Redis suits my needs better (as a high-performance log server; 60k writes per second, trivial replication, etc.)
The problem with Redis is its insane memory consumption. Memory space taken by data is inflated over an order of magnitude compared to its "natural" size on-disk. I'm not sure if this is inherent or is something that can be fixed (eg with some sort of compression) but it makes the DB pretty much unusable for me at the moment. Pity because I really like it!