What's a good Tube Tester to purchase?

[Magnavox300]

Well, after working on three sets now,
I think it's time I got a tube tester.
I have read that tubes rarely go bad, but I was wondering if they get worn?
Do new tubes work better? Or when they go bad, are they simply done-
Anyone know of a good brand tester I should be looking for?
I have seen many on Ebay, but don't have a clue as to what I should get?
I don't need top of the line, just something that will work well.

Magnavox300