LevelDB seems mostly well suited for data that becomes (in terms of key size and number of keys) bigger than your RAM...
I'd be very curious to know a bit about the character of your data, the size of your cluster, etc. (I've only run test clusters at this point, so hearing from someone doing production work would be informative.)
My biggest RAM consumer stores historical data for a goods trading platform. Each trade is a unique key, with all the trade data being the value. Access speed is important, but not as critical as the other goodies I get from Riak (replication and automated rebalancing). Metadata is stored separately, but I hope to change that with Riak 1.0 secondary indexes.