Anyone spending time at the range has had to load multiple magazines, and doing it by hand, especially if you are loading the magazine to max capacity, can get painful on the tumb and other parts of your hand over the course of the day. Before you watch the two videos though, I'll give you the short version.
If you're short on space, don't want to buy multiple loaders for your type of ammo, want something that is compact, mechanically simple, and can be used at the range or at home for loading 9mm, .45ACP, anything in between, or AR-15 style magazines, get either the "universal" (9mm to .45) or M4 / AR15 LULA loaders. There is an optional insert for single-stack 1911 Mags to help make that a bit easier.
If you have room for a bench loader, the Maglula one again appears the best. It only barely beats out the Red Rock Signworks loader in the video below, which I actually prefer how the sorting bin is set up, but the ML is more compact, and has a little more flexibility in what mags can fit the loader. Between those two I'd say it's a matter of preference - and I'd still buy the compact loader for my range back and backpack.