It's pretty good software, and isn't as trivial as some on this thread seem to think (I've built large Solr implementations too, so I know search reasonably well).
The strong points: good interface, excellent data import, decent search language, decent docs & community, good APIs, a good set of mostly decent drop in applications that run on top of it.
The weaknesses: While indexeing is Map/Reduced based and scales fairly well, querying is single threaded. That limits it to the performance of a single CPU core + IO limitations. This also applies to things like sub-queries: in a database they could be run separatly, but in Splunk they aren't.
It is also fairly expensive at large scale, although the licening model is fair (it is licensed by data volume, so you can install it on as many machines as you like and share the license between them).