It depends very much on which variety you get. Green spotted puffers (GSP) like my 3 are not too bad but really shouldn't be kept with other fish which is why mine are in a species specific tank. Obviously each fish has their own personality too.
i should point out that in the puffer tank I have two rummy nosed tetras. I did a big tank swap round a few months ago as I felt that the puffers needed the slightly larger aquarium as their bioload is high (messy so-and-so's) I took all the community fish out of tank one except for these 2 tetras who simply refused to be caught. I then put the puffers in tank one and put the community into tank two.
I spent a further hour trying to catch the two remaining tetra but to no avail. I fully expected the puffers to kill them by the next morning but here we are 3 months later and they are living happily together. I believe that this is due to the tetras being in situ when I introduced the puffers so the puffers simply accept that they are part of the tank. I wouldn't dream of introducing small fish into their tank now as I'm absolutely positive the puffers would despatch them pretty sharpish!