I use one of these for live gigs with my band [I should mention that we use only synthesizers and vocals], and would recommend it for this purpose. Channel 1 is designed to cope with the signal from microphones, and so has more parameters on it than the others. This channel has a standard mic input, as well as a mono quarter-inch jack socket in case you want to use channel 1 for an instrument instead. I use this channel for any synths that give a quieter signal than the others, because it has additional high/low EQ dials that can be used to boost the instrument's volume.
Channels 2 & 3 and 3 & 4 are really pairs of mono inputs rather than four separate channels - this is really a three-track mixer. But it is possible to 'cheat' an extra channel by putting yet another instrument through the two 'CD/Tape' mini-jack inputs. Depress the 'CD/Tape to mix' button next to this, and use the volume control on your instrument to get the right balance. Admittedly, here the 502 isn't 'mixing' as such, as it isn't controlling volume, and you won't be able to pan that instrument right or left, but it at least allows you to get another instrument through the unit, and I've found this very useful at gigs.
The unit itself is very compact. It's light enough for easy transport but sufficiently solid to withstand live performance. If you want a mixer for a band with a few more players, then get the Xenyx 802 instead, which has more inputs. But personally, I'm very happy with the 502. A lot of people have at first laughed at it when I've unpacked it at gigs, only to have to shut up when they realised it could channel our sound perfectly well into a quarter-inch double-jack lead coming from its output sockets.