I had a Nacra 5.7 for about 6 years before changing it for a Nacra 5.2.
Strongs points for the Nacra 5.7:
Reliability, The hull is a bit more beefy than other Nacra
Ease of sailing: Ther base version has almost no adjustements, very easy to sail and very forgiving also
Speed: Very fast for a boardless cat, will challenge many 18' on a beam reach, very fun to sail and very safe at high speeds.
Down points: Heavy, it is an heavy boat. Handling it alone on the beach is difficult. It is heavier than a Hobie16 or a Prindle 18. Probably close to the weight of a board equipped 18' cat.
Capsize: Righting the boat is more difficult because it has a long mast and no centerboats. Righting my Nara 5.2 is easy, rightng the 5.7 is more difficult.
Tacking: This is a more difficult boat to task. The Rick White book "Catamaran sailing for the '90" will help you to refine your tacking technique but is will often go into irons. Suggestion: Turn with a smooth and constant arc, do not oversteer, Sheet in the sail when heading up, make sure that your rig is balanced (not to much helm)
Things to look for on a Used Nacra 5.7
As for all boat, you have to look for the trampoline, the crossbar (look for cracks), the crossbars anchor on the hull and the stiffness of the hull in front of the front crossbars.
The wire halyard will break! about 18" from the sail head shackle. Make sure that you can replace it or repair it.
As for any catamarans, the rudders have to be checked. The nacra rudder system is not as nice as the rest of the boat but it is fery easy to repair (I rebuilt some components myself)
Overall, the rigging is very simple and very high quality.
I still miss mine but I decided to go with a singlehandler.