I'm flying a Science Vessel as of tonight. I flew both a Cruiser and Science Vessel in Beta and decided on the Science Vessel for live. It's got a lot of shields but the hull is weaker. There's some nice tricks with the Science Vessel that I enjoy as well. The maneuverability is pretty decent, but not as fast as an Escort.
All in all I love my Science Vessel. Oh, and in case you're curious, I'm an Engineer.
In the Beta I went for the Escort initially, but then scraped enough credits together to eventually have all 3 classes.
Escort was great, but in a mission where u are outnumbered I got a hammering - being an Engineering Captain helped to a degree, but not enough.
Next I tried the Science vessel as people were saying the shields were awesome and they were right - they got a nerf near the end of the beta but still good and still quite maneouverable.
Lastly I bought the Cruiser - same weapons bays as the science vessel, 2 fore and 2 aft, but man it flys like a pig stuck in tar. I ended up having to load a forward and rear torpedo launcher to get a chance to take out the big ships.
During the Borg encounter on the last day I was in my cruiser and switched to escort - the change in the way it handled was amazing, the escort could spin on a dime but trouble was I got 1 shotted most of the time
So anyway, now I plan to plump for Science vessel again in a couple of levels time. It seems to me a nice mix: fairly maneouverable, with great survivability and when u switch to boost power levels the weapons it still hits hard.