Pros: Good sound quality, two carrying cases, plenty of tips included
Cons: Build quality suspect, picky about fit/insertion depth
I got these about a year and a half ago as a Christmas gift, and they've been my companion for some time until the left earpiece one day began outputting a significantly lower sound level than the right. The build quality in general had always worried me, with the drivers rocking a little in their outer casing, the stress relievers at the earpieces actually coming off and sliding down the wire, and other weirdness. However, they did last me for over a year, and Monster's warranty should cover it.
With that out of the way, the sound quality is rather excellent for an IEM, as is the isolation. With the proper tips, most outside noise fades away, and you're left with good, tight bass and highly detailed mids and somewhat sharp highs. Overall they tend to particularly excel at faster-paced music, such as rock and metal, which also happen to be what I listen to most often.
Comfort-wise, they can be a pain. It seems one of my ears doesn't like having an IEM in, though I had no trouble with the Shure SE-215's I'd been using prior. The plethora of tips that came with them never really seemed to match up with my ear type, and as a result, I'd often have to fidget with them to keep them properly seated. Still, that's more a product of my ears' shape than the IEM's themselves, and Monster did a great job of packing in as many types of tips as possible to work around that. It's worth noting that they are moreso than any other IEM I've used very sensitive to insertion depth, and inserting too deeply or too shallow will negatively affect their performance, from loss of bass (too shallow) to loss of volume or near complete silence (too deep).
Overall, they're a pretty good pair of IEM's, though they'd certainly rank lower with me if they didn't have their great warranty.