The bottom line is that they simply don't offer sound as good as you get from their noise-isolating cousins, the Immersions. Even in a quieter environment, there isn't quite as much bass and they're not as detailed. That said, they still sound decent and measure up to competitors like the Bose SEI2i series and cost $50 less (though the Bose models are arguably slightly more comfortable).
Monster says the Intensity earphones are "tough, durable, UV-protected, and sweatproof for your most aggressive workout." In my tests I used them for a week, running a little outdoors but mostly on a treadmill at the gym. They held up fine over that time and overall I came away most impressed by their fit. It's hard to find sports earphones that slip in and out of your ears easily and also manage to fit securely. These have both those attributes.
It's worth noting that they sport a tangle-resistant flat, linguine-style cord that terminates in a sturdy L-shaped plug. They also feature an integrated ControlTalk remote/microphone that's a Made for iPhone certified product, geared to users of Apple's mobile devices. The microphone features should work with other smartphones but the remote features will be more miss than hit (read: they won't work).
When reviewing sports headphones, design can end up carrying a little more weight than performance. The Monster iSport Intensity In-Ear Headphones' design clearly has its pros and cons. If you prefer not to jam silicone eartips into your ears and are looking for sports earphones that allow some external noise to leak in, these are an excellent choice. They offer a comfortable, secure fit, are easy to slip in and out of your ears, and seem fairly durable. Their sound quality is decent, particularly in quieter environments, but they don't sound quite as detailed or offer as much bass as Monster's noise-isolating sports earphones.
I can't say the iSport Intensity earphones are a steal at $99, but they're less expensive than Bose's similarly designed sports earphone models and they're not grossly overpriced. That may not sound like a ringing endorsement, but I have no problem recommending them, particularly to folks who are looking for a more "open"-designed sports earphone. Unless you're expecting incredible sound, you shouldn't be disappointed.