The MicPortPro is fine...for a backup. I used it in my makeshift bedroom studio last year when I was convalescing from surgery. It worked fine (although I occasionally got a few random electronic "pops" in it that caused me to have to recut a few things). Otherwise, the sound is certainly more than acceptable.
But I wouldn't consider it a "primary" preamp.
But in the event I'm ever on the road and need to pack my laptop and a mic, the MicPortPro is the ideal pre-amp for those situations -- light, doesn't take up much space, and sounds pretty darn good.
I have 3 MPP's, and use them daily (well, I keep one in a backpack for backup/mobile use) with my own Mac/Logic-based VO studio. Really great for the price, often edging out similar units by Blue, MXL, and Shure in reviews (Shure's comes closest in features but doesn't go up to 96k, a non-issue for most VO work). Phantom power, portability and mic/headphone level controls on the unit make this a no-brainer for most VO types, at home or on-the-go.
99% of the time the sound is great, but not faultless, as Randy's pointed out. I too have experienced random, take-killing static-like pops, and occasionally, if I leave one plugged in for a long time, I get random high-pitched whistling noises that CEntrance support was unable to definitively figure out. I've experienced both issues on all 3 units, on both my main iMac workstation and on my MacBook Pro, and am still trying to determine who's world it's rooted in; there doesn't seem to be many other reports of this out there with the MPP, but there are reports of similar probs on the Apple forums with other Mac/audio hardware combinations with USB connections involved.
A similar but more expensive option to look into is Apogee's One (USB/single channel) or Duet (FireWire/2 channel) units. Cool features, stellar reputation in the audio world, and probably one of the most Apple-oriented/integrated audio manufacturers out there, and everyone I know who has one raves about it. I've seriously considered using both, but the MPP's single-cable connection integrates faster and better in my particular studio situation for now.