We found this trip to be the highlight of the hoilday. We went hiking for 3 days in the mountains, camping for the nights that we were staying in the Park. We got Sammy Kasaw to arrange our trip for us (tents, ranger, guide, food etc) and found it very good.
I would advise going for a few days as there is a lot to see and it also takes a few hours to get there from Gondar. If you try and see it in one day you will not get to see much of the scenery or the wildlife. Our cooks were very good, even though it was easter week and they were fasting themselves, they kept us well fed.
It does get quite high up at parts so while its not very difficult in terms of hiking terrain, the height can cause some issues if you are not used to it.
All in all definitely go if you are in the area, give yourself a few days for it, get a good guide to set you up with tents/accom/food/travel etc. and enjoy one of the most picturesque hikes you'll see in Africa.
As an aside, our guide was excellent and I would highly recommend him if you are looking for someone to take a trip with. You'll find him at hikeethiopia.com, Sammy. Very knowledgable and friendly, just tell him Conor from Ireland sent you on :)