Nacra 5.7 / 570's are really fun boats. If you are not looking to race, but want a fast boat, Nacra 5.7 is a great choice for a couple of reasons:

1. Boomless, simple rig, easy setup.
2. Furling jib easy to de-power.
3. Bigger to take more people out with you.
4. No daggarboards to worry about.

The guy in the slip next to me has a 5.7 and has single handed it on moderate days (7-12mph wind). Otherwise, two people is the normal for guys our size (160-190lb). Remember it is at 19' boat.

If you are not going to race... look at a Hobie Getaway. My dad single-hands his 80% of the time. $6000 new, worth every penny.