We've been using OpenTSDB here in an extremely high traffic setup for about a year now, with absolutely no issues at all. It took a few hours at most to setup and figure out scaling.
If you have a large team of engineers writing data into it, they will sometimes abuse the schema by overloading single metrics with many tags. Every permutation of tag*value creates a new row per hour of data. This creates extremely hot shards when engineers inevitably do something like store a client IP address into a tag. When this happens, you may have to write some web-UI scraping code to figure out which regions are getting the most traffic (assuming you have "extremely high traffic setup" numbers of region servers), or script up something like misra-gries on a tcpdump of its inbound metrics to see where the hot shit is so you can get somebody's deploy reverted. I know of some companies that have forked OpenTSDB and prefixed each row with a hash such that it spreads overloaded metrics around the cluster much more evenly. KairosDB solves this by not using a lexicographically sharded database (Cassandra).
When run properly, it runs great. But that takes some really painful learning experiences to learn how to do, generally.