you have to partition your SD card the correct way in order for it to be used. Please make sure you understand what the difference is between ext2 and ext3 partitions.. then understand the risks involved in using app2sd.
App2SD is a great idea...in theory.. in actual practice it can go horribly wrong. if you don't choose the correct partition type and the card is removed while powered up...or you power up with the card removed, you can brick the phone (its happened before).
Even if you do choose the correct partition type, and remove the card while powered up..or power on without the card present, the phone stops working. in most cases you can simply replace the card and reboot the phone to correct it.
But what happens IF your SD card fails? You have a basically useless phone at that point. If you transfer any system apps or system cache to the SD card and it gets removed or the card fails. you brick the phone.
once you fully understand ext2 and ext3 partitions and how to partition the SD card properly, and understand the risks involved in app2sd.. go for it..