The problem with NiMH AA and AAA rechargeables? Relatively short shelf life for the charge. Alkaline cells, for example, hold their power for years, while NiMH's loose up to 3% of theirs per day (depending on ambient temperature).
The good news? A new generation of "hybrid" AA and AAA NiMH rechargeables are claimed to offer the best of both battery breeds. The Rayovac Rechargeable Hybrids and Sanyo eneloops, for example, are said to have self-discharge rates that rival alkaline's, and, like alkalines, are ready to use out of the package. Like NiMD's, though, they can last up to three times longer in a digital camera.