GTAIV is poor in comparison to SA imo! There is nothing to do! I was bored within a couple of hours. First time this has ever happened on a GTA game. So much has been removed from IV in comparison to SA, money for example is pretty much useless as you cant buy anything! Cant mod cars or buy property, no push bikes, pretty much no character customisation, no jet pack, no planes, I could go on. The game sucks in pretty much everyway apart from the GFX and the phone.
On the whole if you look past the frame-rate issues GTAIV is worth the play but in comparison to San Andreas it royally sucks.