Vola's decent for fulfilling file requests in active threads and for hypno grab-bags thanks to its ease and high speed, but it's awkward for trying to download whole collections and requires constant re-ups to work. It worked okay before a few of the big rooms got hit by copyright and it's been a bit of a mess since then.
Before that, Giga was great for mass-downloading and allowed for easier file finding thanks to folders and shit, especially with the ability to sort by latest uploaded. But unlike Vola, stuff wasn't out in the public and you had to build up your own network, hoping that some of the big sharers would see and accept your request. Plus you had to sift through all the files that you had seen on a dozen other users beforehand to find previously unseen content.
MEGA's sort of a great middle ground, especially with the ability to import it to a cloud drive and download later, but a download link can't be updated later to include any more files, and there's no easy way to aggregate and find files other than going through a bunch of different links. Plus it's been shown to be fairly susceptible to copyright strikes.
I haven't created a retroshare yet, although it sounds like a better form of Giga. Although history suggests that it'll last for about a year before imploding and people turn to MEGA or fucking ufiles before finding the next great way to share.