RWGAST, it's difficult to give you a simple yes/no answer to your question "Are these cheap power supplies worth the money?" since i don't know how much you pay for your money!
Personally - i would not trust these supplies, but i am quite willing to accept that my sceptism/caution may be over done. You must decide.
As is noted elsewhere (and I concur), you cannot tell the quality of these supplies from the outside. Many fair and appalling supplies will even use apparently the same outer front panel plastic moulding, but have radically differing quality interiors. This will even change over time from teh same supplier, as they swap and change their own component suppliers.
Power supplies can fail in a number of exciting ways. The bigger the supply, the more potential excitement! Often, the main output transistors will go short circuit, so dumping maybe 40V DC at up to 8 amps across your load. If that load is only a heater or motor, it might even survive that. An Ipod will not, however!!
At the end of the day, you do get what you pay for. Cheap price will invariably mean cheap quality components and/or construction. You might getter better value and more reliable results from a branded second hand power supply? eBay is awash with those.