Les Trois Vallees consists of La Tania, Meribel and Courcheval (although I sometimes hear differences of opinion as to exactly what they include). My parents own a place in La Tania so I've been there numerous times and it's fantastic.
In terms of skiing, if you are a beginner a simple lift pass (eg just a La Tania pass if you're based in La Tania or just Courcheval if you're in Courcheval) is definitely enough, but if you're a very experienced skier you will probably want to pay the extra bit and get a 3 valley pass. While some ski resorts are mainly just good for beginners or just good for advanced skiers, I think the 3 valleys is good for everyone - a decent amount of green runs for complete beginners, lots and lots of blue and red runs, and then some blacks too and a ski park for those more adventurous who want to start jumping. There are definitely enough runs to keep even the more advanced skiers busy and to make you want to go back year after year
The conditions are normally great from around January once the first snow has settled, until April when the season ends, although January and Febuary are probably the best times to go for snow as it can melt towards the end of the season. Even in Jan/Feb you will get lovely blue skys and sunshine so it's my favourite time to go.
Regarding the nightlife, La Tania is small with a friendly atmosphere and only really 4 bars + a hotel bar. The main pub is called Le Ski Lodge and has live music both sometimes at apres-ski and in the evening. The bands are normally regulars, such as Bring Your Sisters who are fantastic and will do really popular covers and get the place buzzing. There are also a few different restaurants to choose from as well as eating pub grub in the ski lodge which after a long days skiing is normally what we go for and it fills a big space and is very satisfying. Meribel I don't know that much about but Courcheval is the place most people have heard of and has a lot more nightlife and clubs but is more expensive.
All-round it's a great holiday and I could go into a lot more detail but it would then become a brochure rather than a brief overview!
Les Trois Vallees resort, in the French Alps is accessible from Chambery airport, for skiers. However, the airport should be avoided on Saturdays if at all possible (usual day for ski changeover) - the luggage carousel simply isn't designed for 300 pairs of skis and boots as well as suitcases to arrive in a two hour slot - you'll spend an hour searching for your stuff!!
However, the misleadingly named 3 Valleys (it's actually eight resorts now!) is the largest interconnected ski area in the world - you can ski over 600km worth of pistes.
In recent memory, the snow has been reasonably reliable, although the lowest resort, St-Martin isn't always so great, and there have been days in the coldest part of the season when runs/lifts have been closed due to adverse weather.
That said, it's an amazing place to ski. There's huge variety in the type of skiing you can do - powder, groomed/ungroomed, glacier, off-piste, etc, and you'll never be bored. The lift pass covers all eight resorts, and it is possible to do the "3 Valleys Challenge" - skiing to 13 points in the 8 resorts in one day!
There are snow-parks for snowboarders (if only they'd stay thee...) and great apres-ski, including the highest pub in Europe - the Frog and Roastbeef, and a club whose name we never discovered, but which we affectionately named "The Sweatbox".
Recommended for fun, younger crowd, beginner-intermediate.