I've been to Whistler twice. The first time was in April so here is my experience. I also wrote an article talking about Whistler (and other resorts) so maybe it will help. Whistler
The first time I went, it was during the Telus Ski and Snowboard Festival. I didn't plan on going during that festival it just so happened I was there. If you're going at the same time, you'll have lots of options for partying, special events, etc.
Usually in April you can find some great deals to stay in Whistler village. We got a 1 bedroom loft in the village for two nights. It was about $300 (3 people) and we got 3 2-day lift tickets also. Check the Whistler website for deals. If you can't find anything affordable, try vrbo.com for rentals.
Typically after March, they close Blackcomb mountain (which sucks as I think it is the better of the two). They prepare it for their summer skiing which starts in May I believe (and then they shut down Whistler Mountain).
The top of Whistler will have great skiing. Wide-open bowls and steeps along with blue cruisers. Watch the shade early in the morning as it will be all ice. Once the sun hits it, those runs will soften up very nicely.
The middle part of the mountain will be hit or miss. If it is open, it will be like spring conditions (except for the shady parts) around here.
More than likely you will not be able to ski the bottom of the mountain. In fact, it's tought to do that in February as it can be slushy even then.
On my April trip, it was high's of 60's and lows of 40's at the bottom of the mountain around the village. It was high's of 30's and low's of 20's at the top.
Don't be discouraged by the temps at the bottom of the mountain when you get there. Once you go up 5000 vertical feet, things change a lot. When I went in February, it rained everyday at the bottom of the mountain. It snowed everyday at the top (depending on which side of the mountain you were on).
Once you get there, you do not need (or want) a car. In the middle of Whistler village, you'll find the grocery store, liquor store, and drug store. So you'll have all the necessity's available to you. Restaurants, bars, shops, etc. all over the place. This is the jewel in Intrawest's stable.