The bundled card reader works for most people, but a lot of people have gotten ones that don't work, and a few people have had melting type overheating problems. There have also been many reports of intermittant data/filesystem corruption caused by the bundled card readers.
On the bright side, the reports of card reader problems have tapered off somewhat, and there are rumors that they are including more reliable readers now that they did originally.
I say get a good card reader if you can afford one, especially if you don't have to mail order it to get a good price. If you can't afford one, the included reader will probably work just fine. Just keep a close eye on it the first time you use it. If you do get the rare melting reader, you want to find out and unplug it before it destroys your microSD card.