I've had a Necky Gannet II for almost 5 years now. It is very versatile, being able to switch between tandem and solo paddling quickly (The forward seat slides aft to paddle solo). It is great on ponds and nicer days out on the Bay. Even took it down the West Branch of the Penobscot, and it handled the rips very handily.
Positives: Stability - It is a very stable boat. This is nice on moving water, paddling with inexperience folks or a dog, and when there is something interesting to look at below the surface. I've even stood up to board larger boats.
Versitility - We've used it on quitewater to class 2+. It does great. Solo or Tandem.
Negatives: Tracking - I don't think it tracks very well. I think this can be attributed to the 'full' bow, which is great for surfing (doesn't dive). Maybe the rudder would help.
Wettness - the boat is pretty wet even in small waves/chop. If you're paddling tandem and you're sitting in the stern, its fine, but the bow person gets soaked :) A spray skit is recommended.
It has been a nice boat for gunkholing and paddling on quite ponds and even quick moving rivers. I think of it as a combination of canoe and kayak.