My first bought skis were the k2 escape 5500 which was the precursor to the 5.5. The K2 5.5 is a great value ski. I was recently back on my 5.5 and had a lot of fun. They are actually a good bump ski because if you have to you can easily skid the tails. That said here is the downside;
K2 doesn't give much rebound, the ski doesn't give back much energy. It is very stable and will go at pretty good speed unless you really want to fly like a bat out of hell.
The easy skidding has the downside that if you want to really learn how to carve, you have to concentrate to not let the tails skid. This can slow progress you may get on a less forgiving ski initially. These will carve though with the right technique.
A better, (more expensive ski), may grow with you for a longer time frame.
That said, I really enjoyed my skis and they really helped me move from novice to an advanced intermediate. They are competent on black slopes and will even go into 8" - 12" powder well. The biggest thing I notice is the difference in energy/rebound. If you like rebound, you may not like the K2. Peter's rating of 3 for everything and 4 for forgiveness is fair. They don't shine at anything but do everything adequately. Basically comes down to passion and budget.
Perry aka skidaddle