That's a tough question. If you know what fish/plants/inverts you are interested in, you're likely to get a more meaningful answer, but here are my thoughts.
The kits are usually suitable for a fish only setup--they don't have covered intakes to prevent shrimp from being sucked in, they lack the necessary light for all but the most basic of plants. There are exceptions, of course, but mostly in less than 10 gallon range.
Another consideration is that many of the kits include press board stands. Press board is sturdy, until it gets wet, and then it becomes a slowly sinking time bomb to flood.
Personally I wouldn't buy a kit for a 29. (well, nor would I buy a 29, but that's a different issue entirely). The filters that come with the kits, even if suitable for the stock you choose, tend to be on the lower end of quality, and wouldn't meet my needs. Ditto the light. The heater MIGHT be ok, but since most of my tanks run without heaters, it's fairly moot. Optional items such as 'bacteria starter' are a waste of money, and the water conditioners are usually not my preferred brand and all too often include additives I avoid (ie, Aloe. Fish do not need aloe.)
