Right now, I'm working on the previous version of the display; the LED Display. I had a Thunderbolt Display -- ordered it the first day I could -- which worked flawlessly up until a mother nature issue (ironically, it was damaged by a nearby lightning strike). I bypassed replacing it with the same model -- this was August 2013, and I thought that surely a new Thunderbolt Display was about to be announced. The TB Display was originally released in 2011 and, with the pending release of the Mac Pro, surely Apple would release a new model/version.
Nope.
We're now over 1000 days since the release of the Thunderbolt Display with no update in sight. It's still got USB2. Still got FW800 (good for many, I'm sure; I've had to hook up a legacy device once or twice).
To answer your question; it's reliable. Like I said, I'm working on the version released prior to the Thunderbolt Display, the LED Cinema Display. It's a little dimmer than I like but it still works w/o issue. My previous model (going two Apple Display purchases back), another Cinema Display, is still in use by a friend without problems. And my display before that -- the 30" Apple sold some time back -- is still being used by the friend who bought it.
Their product builds are top-notch, and they insist on the cream of the crop from their suppliers. Apple makes excellent displays. When they feel like making them.