Some Bads:
As implied above, there is heavy reliance on rp penalties that could be hard to interpret or work into play and were usually ignored.
As admitted by people who worked on 2E, there was power creep, and basically had to be to get people to buy and use these books.
As with so much in 2E (ok RPGs, ok ok life in general) these were very much made up as things went along and so you have big differences across kits.
The Good
With 20/20 hindsight, kits were a simple and sometimes very effective way to customize charecters. The fighter ones, which were first, made the class far more versatile and made it easy to play a rang of archetypal charecters. As noted by Crothain, the Bard ones also really opened up that class.
On a user note: Player Option: Skills and Powers (note, use this book with care!) has a complete list of kits that may be a little more balanced with each other. But I am not really vouching for it.