Easy to use - I can see what's happening without having to click into my browser or open an image previewer just so i know which way round it needs turning! The only thing about Fancy box as far me personally goes with using this service was their charge at first (free trial) but after doing some research other services have followed suit by offering free trials instead! So if you want your users/customers experience improved then go ahead give them FREE TRIALS.
It also makes life easier when sending out newsletters etc where people aren't always going through all emails carefully they will get distracted from more important things once something pops up!!