Here's how it works:
1 day pass is valid for 24 hours. If you get it at 5pm, it's good until 4:59pm the next day.
You have the bike for free for 30 minutes. All you have to do is put the bike into another rack before the 30 minutes is up and grab another one right after. It's free all day long like this. If the rack is full, just put in your info in the machine and it will give you an extra 5 minutes to find another rack. They're everywhere.
If you have an iphone, I recommend downloading the Velib app. It shows you where the stations are, how many bikes and how many open spaces are available at each rack.
Finally, they do hold $150 on your account but it's not per bike removal. If you remove a bike 10 times in a day, you won't be charged $1500. Now if you have 3 passes on one credit card, you'll probably be charged $450. Regardless, you get the money on hold back quickly. I've never even seen it on my credit card.
