Cleaning up other researcher's poor data is terrible. There's not much in the way of standardization, and the software is pretty terrible and not at all user-friendly. They quality check the data, but from what I can tell, it doesn't take much to pass. Some people obviously take advantage of this and do just enough to get by. After all, if the company is a thousand miles away, how are they going to know? You can kill yourself trying to get all the data perfect. Again, patience is a big help here.
As far as the field work, it can be kind of frustrating and intimidating. People have no idea who you are and get very suspicious if they see you snooping around writing down tenant names and taking pictures. You can explain what it's for and that CoStar actually helps the building management out a lot. Some people will be cool with it, others will call security and kick you out of the building. It probably gets easier to deal with after a while, but temporary employees aren't very convincing to suspicious people. If you can "sell" the service to the property managers, you're more likely to be welcome, and they might even help you out by just giving you the rent rolls.