I have one in my game. They are 7th level, and she is a newer player, so she built the character around using spells as utility (pass without trace, cure wounds, good berry, etc) so she could focus on hitting stuff with two longswords and her kitty (a panther). They collectively do a LOT of consistent damage every round, albeit it she often finds herself falling behind if the creatures are damage resistant to non-magical weapons of if her cat is not there (knocked out, gone, etc.). I'd say they aren't in a bad spot all together, not too overpowered, but certainly stronger than the base book (which was a tad weak). So far she's having fun but hasn't really taken too much of the spotlight, short of when it was appropriate to do so from like character related stuff or scouting or helping the party avoid random encounters.
Honestly? The hardest part has been dealing not with her damage potential (she knows her cat is a glass cannon and I WILL kill it if she gets too cocky), but with stuff like natural explorer making traveling trivial for the group, or the roleplaying, such as bringing a tiger into a town, etc. Primeval awareness is the one ability I would like to see tweaked, preferably so they know the direction of favored enemies, but not exact number and exact location. That said, she's not been doing stuff like insisting I tell her about the exact number of squirrels and butterflies in a 5 miles radius, so it hasn't really been too bad in that regard. >_<
I should probably also clarify, I houserule that her pet acts on her turn to avoid confusing initiative orders. I tend to do this with all summoned critters as well. I just let her use her pet's initiative modifier or hers, whichever is higher for checks. It might make things a bit exploitable in the hands of a powergamer, but it's usually not that big of an issue for my table, and I find it makes the game run quicker.