I've been paddling for about six years, mostly polo and occasional whitewater up to grade 4. I'd consider myself a competent class 3 kayaker. Our club has a 3D that I've paddled on Goolang creek, which is fairly tame with rapids up to easy grade 3 lots of nice small waves and stoppers to surf and holes to play in, and the Upper Nymboida River, with large grade 4 chutes and drops up to 3 metres, and numerous shallow boulder garden type rapids. I've also taken it out in ocean surf in a fairly gentle swell. I found the 3D to be a great beginner's playboat on the creek, as its medium chines and planing hull allow you practice your surfing and carving skills, and the low volume stern makes tail squirts a breeze. It'll cartwheel pretty easily in a decent hole too. I first learned to surf and do enders and squirts in a whiplash, and found the 3D much more playful, but more forgiving than modern playboats with very hard chines like the Riot Tricktser. It was a different story on the Nymboida though. It handled the drops and chutes ok, but the flat stern and pointy bow made the 3D difficult to manouver through the rock gardens. It has virtually no rocker, so the bow tends to dig in very easily, and can flip you very quickly if you are caught leaning against a rock. The stern also tends to catch easily. I had the same problem in the surf, and found that I needed to lean right back over the back of the boat to keep the bow from pearling while on a wave. This wasn't a problem in the trickster in the same conditions.
So, overall:I'm a IV boater. 5'9", 175 lbs., 32" inseam with 9 feet. I agree almost to the letter with Brian Landers entry below, only I've found the 3D to work pretty well in Class IV. It is certainly not a Class V boat, so I guess there is a line that needs to be drawn somewhere.
Pros - It tracks well on flat water and is easy to roll, even hand rolls aren't too diffcult. I'm about 6 ft and 80 kgs with size 11 feet, and I can fit comfortably in the 3D thanks to the foot bumps. Surfs and spins easily thanks to the short length and planing hull. Carves easily on smooth waves. I can cartwheel it easily in a hole, but haven't managed on flatwater.
Cons - I'm considering trying to modify it to give it more rocker, as the lack of rocker is the biggest problem with this boat in my opinion. The pointy bow sucks. It digs in way too easily, making boofs and steep waves needlessly difficult to handle.
It aint much of a playboat, though it surfs and squirts very well. If you're my size or smaller, pick one up as a river runner, and you'll be happy.