Pros: Plenty of skiing
Cons: Busy in high season
Let's start off with the skiing. There's lots of it and the rating of it is, um, variable, most notable are the greens off the Col de Fresse lift. The only bluer green I can think of is Big Emma at Snowbird. But enough about that, my advice would be to ski the runs, and ignore the colours (If you normally stick to reds and blacks, try a few of the greens and blues - you might enjoy them)
Grattalau on the Tignes side is a great place to start the day. It's a short, wide blue run. One side is always pisted, the other can be a powder field in the right conditions. The run down to Val Claret from there (Carline) is easy at the top, but the last pitch of it is steep and can be quite icy.
The cable car up to the top of the Grande Motte is worth doing if the sky is clear and the wind isn't too strong. Double M, the run back from the top is a long and tiring red, but in the right conditions it's great fun. One other key run into Val Claret is Piste H. Another run that can get icy in places, so ski with care. The final run I'd like to highlight on the Tignes side is Anemone. You can get to it from Grand Huit, Aguille Percee or the Palafour lift. If your friends want to stop for a drink in Le Lac, this is a great run to do a few laps on.
Now over to Val d'Isere. If you've ever watched the skiing world cup, you'll know some of the runs into La Daille. Do them, but they can get crowded and icy. They also lead to possibly the ugliest architecture in France. If you are a reasonably proficient skiier, the run you really want to do is Face. It's steep, it's long, and there's no easy way out. The other way down from Bellevarde is Santons or Epaule. If the conditions aren't good, avoid these runs as they are not nice (unless you are on rental skis).
Keeping going to Solaise and head up the Glacier Express. From there, if the Tunnel is open, you might want to try it, but it's not that easy. The other option is the Leissieres Express, or roller coaster. You can't get off it at the top - it goes over a peak and down the other side. Not for those with a fear of heights.
This section can be the quietest and the most fun, and is worth spending time in. Ski all the way down to Le Fornet, or stop half way down for food at the gondola station.
This leads nicely on to food. As a rule of thumb: Val d'Isere is more expensive than Tignes. On the Val d'Isere side, I would consider the restaurant at the top of Solaise or the one mentioned above. Otherwise, save your money and eats in Tignes. (I recently bought a small soup and small beer at Folie Douce for about $20)
So, in Tignes you can eat up at the Grande Motte, but I wouldn't. The places I would recommend are the Taverne (beside the Bollin/Fresse lift in Val Claret), which does great hot chocolates. Next would be the Aspen restaurant. Good value and very tasty omlettes, burgers and goats cheese salads. Big portions too. My other favourite stop is the Rendez-vous in Le Lac. Very good, fresh pizzas (although the lady who runs it has an interesting dress sense).
Tignes wins for value of food, but Val d'Isere probably beats it for nightlife.
I could write more, but that's enough to start!