Had a buy one get one free coupon and it was during the day... so it only cost around $10.... but we waited in line for about 30 min., only to go to a small observation deck. It was an impressive view, but one that could be obtained from a hotel window.
If you have a coupon and some free time it may be enjoyable. However, this is something you could skip. Spend the time just walking around the Paris hotel lobby.... as I found that more exciting and it cost nothing.