I've been to Crobar about 10 times over the past year. Its a nice big open night club. The only bigger nightclub in Chicago that I know about is Vision. But I like Crobar more. It get very busy on Saturdays, and seems to have had a change in ownership (or promotion stragety) not too long ago, because its popularity rose dramatically a few months ago.
The only thing I don't like about Crobar are the long lines after 10:30 on Saturdays. You sometimes need to stand half an hour in line there... And what makes it worse is the fact that they only take in about 3 people per minute (super slow). So even if there are only about 50 people in the line, you are still stuck for 15-20 minutes. Unless you have a reservation or are on a vip list, of course - then there is no wait or a maximum of 5 minutes. Their bouncer (big black guy) that controls the line and checks id's seems to only treat his friends and cute girls nicely. Others are out of luck if they try to talk to him - he just ignores people (perhaps, its something that comes with the job). I guess he's allright, its not like he says to go f**k yourself.
Inside is a single room with two levels (second level runs around the perimeter of the big open area and has the vip area with 2 abrs and one more bar bar). There are tall ceilings, a very good sound system. When it gets busy, the dance floor is full of people that are actually dancing, but at the same time there is still enough space to walk around. As I remember, there are 6 bars total in the club (2 of them are in the rather small vip area). Bartenders are nice. Drinks are expensive (except on Fridays I think they have 1 dollar drinks, but really bad drinks)- jaeger bombs are $11, redbull vodka is $12 - these are the ones I got last time and still remember actual prices of.
Crobar nightclub in Chicago definitely seems like the place to be in on Saturdays if you like clubs and house music. Its nice inside, the crowd is rather friendly and seems to be mostly below 30. So I can say its a younger crowd night club in downtown Chicago (with 3/4 of people appearing 23-30 years old, and the rest younger or older).
They have valet parking and a parking lot between Crobar and VIP Club that could take your car for $10 or where you can park your car right at the front for $20.
Next to Crobar Nightclub (which is right off of North Avenue, east of the river) are several other bars/clubs with very different crowds and catering to people with various interests and musical tastes, so even if you are not a big fan of Crobar-type clubs, this area is likely to have something you'll like anyway (if you like going out in general of course).
Bottle service at Crobar (added 1/20/2010)
I'd suggest not getting bottle service there if you have any kind of "limited" budget. They add sales taxes to the bottle price and also a "service charge" of over 10%. Red bull is not included (8$ per can extra, I think). Tip is also not included in the bill. Thus, when we ordered a bottle that was supposed to cost approximately $175, I was very surprised to a get a bill for $230 which also needed tips to be added.
I just think that this type of hidden charges is a bad practice - clubs never charge any sales taxes on top of their prices and definitely don't do service charges. So when they say a price for something (I got the $175 over the phone), you expect to add only tip to that. Just keep these hidden charges in mind when getting bottle service at Crobar in Chicago.