I have allowed in in my own game and now have two PC's with the feat. The caveat to taking the feat was that cohorts must be recruited from an 'existing' pool of NPC's that are generated by myself. I also reserve the right to take over the cohort and run them as an NPC at any time. This also goes for leveling up, for which input from the PC can be given, but is sometimes not taken. Like many others have posted, cohorts have their own motivations which can make for fun complications.
Overall they have been a positive influence on the game, but they do slow combats down a bit. They tend to be more fragile than a PC (28 vs 32 point buy's) and are often relegated to support roles, AKA run and hide until the fight is over.