Honestly, you can't walk around a lot of the site as it is still being actively researched. But entrance is pretty inexpensive (Expect 10 soles, so a little over $3) and includes admission to a small museum and zoo with little animals. You can get an informative guide too. My archaeological opinion is that the site is really worth the look, but then I am a completely biased Andean history dork.
The restaurant is upscale. I didn't really know this when I went, so I showed up looking like your typical dirty American backpacker - grubby clothes and a baseball cap. And then I saw that everyone else was in suits and cocktail dresses. Dammit.
But they let us in anyway, which turned out great since the restaurant is excellent. The atmosphere is cozy, with lots of wood and adobe finishing and big fireplaces. You can even sit outside on a patio overlooking the site. The site is closed at night, by the way, so you'll have a quiet and unobstructed view. Plus, they light it up beautifully. Being able to eat dinner while looking at a 1800 year old pyramid with a spotlight on it? Win.
When you go expect to stay awhile, eat several courses, drink some wine, have an entire legion of waiters offering to fill your water, and enjoy the view. The staff was so polite, even when faced with a troop of under dressed gringas. Some waiters do speak English, and they also have English menus.
As for the food, it was very good. Just don't expect traditional Peruvian food; it is Peruvian food like P.F. Chang's is Chinese food. I had the tamale appetizer, which was big enough for a meal. Being from the Southwest, I'm a tamale snob - and this one was damn good. I also had some of the gnocchi; it was rich but delicious. Everything else looked really good too, but keep in mind that it is all pretty pricey compared to most other places.
Overall: It has good quality food and great waiters. Is it the best food I've had in Peru? No. It also isn't where you want to go if you want traditional Peruvian food. However, it is really hard to beat the romance of the setting. It might be one of those end-of-the-trip meals where you've already had as much ceviche and papas ala huancaina you can eat, so you just go to relax and have decent wine. And stare at a giant awesome pyramid.