I've never administered it, but I used it as a support engineer for a year and a half, and I have nothing but good things to say about OTRS. Compared with the three other ticketing systems I've used, it blows each one out of the water. (Pretty much every ticket we were working was email based though, so your mileage may vary if you have a lot of 'phone' tickets.)
Sorry I don't have a lot of technical know-how to impart about the system, but using a tabbed browser and OTRS at [insert large Unix company here] made life a _lot_ easier than it could have been.
Suffice to say, I wish I was using it at my current workplace.