I also had a little trouble getting a really tight seal. I tried about every tip type included and got a fairly tight seal (I ended up using the medium silicon tips instead of the the foam ones that I usually prefer because the foam ones just didn't sit in my ear as well). For many people, especially for those who want to be able to let in a little bit of the outside world as they exercise, that won't be a deal breaker, but you will lose some bass if you don't get a really tight seal.
In the plus column, the L-shaped headphone jack is well designed and very thin, which means it plays well with thicker iPhone cases and fits into recessed jacks. That's important for some folks.
If you get a good seal, the UltraFit 3000s sound quite decent, with reasonably detailed sound and an ample amount of bass. It's not great sound but overall these guys are pretty balanced and a step up from the standard issue Apple earbuds, even the new, improved Earpods version Apple has included with the iPhone 5 (the Polks will also stay in your ears better). The UltraFit 3000s are tuned for mobile products, so you should have no problem getting plenty of sound out of them (they played plenty loud for me at about 65 percent volume).
As with most in-ear-canal headphones, the key to getting the best sound is how tight a seal you can get. In other words, performance will vary according to that seal, so some people will feel better about the sound of the UltraFit 3000s than others. Overall, however, they do not measure up to the Monster iSport Immersion in-ear headphones in terms of sound or fit.
The Polk UltraFit 3000s are a product that's going to work better for some people than others. While some fraction of buyers will have a harder time getting an excellent fit and won't be impressed with the sound or the comfort level, many others will think they offer a very secure fit with strong sound.
While I didn't love these earphones, I did like them and appreciated all the included accessories, which boost the odds that you'll achieve a proper fit. In short, the UltraFit 3000s are good, just not great. At $99, I might be a little hesitant, but now that they've come down in price, they're easier to recommend.