I won't go so far as to say that all microSD cards are alike (if they're like their bigger cousins, there are differences in reliability and speed), but apart from those who have specific needs (such as recording a burst of pictures in RAW format), most people can simply buy on price. A friend of mine bought an 8GB microSD card (with full-sized SD adapter) for his BlackBerry on sale at Fry's for about $20.

If you plan on reading the card in your computer or some other device, you might want to make sure the other device also supports the higher-capacity (> 2GB) SDHC cards and not just SD cards (similar to the difference between MS Pro and MS).