I looked at some reviews for this recently, and they were pretty bad -- I guess the batteries can't be replaced, so you'll get a pretty short life out of the thing.
I've had my tempo trainer for about 2 years now and haven't had any problems with it (or the battery).
I use mine mostly for pacing long swims or when I need to practice sprints to help me pick up the pace. For me, speed it entirely mental and this keeps the rhythm in my head.
Tried it tonight... 90 was my goal, but hard to maintain after 200 yards... 105 very comfortable. No idea what is "good" tempo... Guess really an individual thing like Bike/Run cadence. Any idea what is typical for good swimmers?