I own
the ME50 - I am very satisfied with it (but it took me some time to dial in sounds that I liked). The only guitar effect I had before was a Zoom 505 - so the ME50 was a significant improvement.
Guitar is not my main axe, so I did not already own a bunch of pedals. When I priced out individual pedals, I figured the ME50 would cost about the same as 3 or 4 pedals (and the ME50 has so many more options). I already have some decent guitar amps as well as a couple of amp simulators, so I did not need a multi-effect with amp simulation.
I wanted something with knobs, because I hate scrolling through a dozen "windows" to find a parameter. The ME50 reacts much like any Boss stompbox - you can quickly tweak a knob (vs. scrolling).
Since I have not owned all the pedals the ME50 simulates, it is hard for me to judge how accurate the simulations are, but I can dial in just about any sound I need (it gets easier the more I use it). At first I thought it sounded too "harsh" but the more I tweaked it the more I was satisfied.
I too thought the cost was too much for my limited expertise on guitar, but I knew I needed compression (sustain) and distortion and some time effect (delay, chorus, etc) and by the time I bought all those individual pedals, I would still be spending over $200.
When I started with the ME50 I considered myself a sound novice (I've learned plenty of guitar licks, but I had not mastered sound) - but little by little the ME50 is helping me understand what effect does what toward getting the sounds I need.
Obviously, tone comes from the guitar, the amp and the fingers, but sound comes from lots of little bozes (or in the case of a ME50 - one big box).