When babies start to pull themselves up, the cot should be lowered. You can still have the mobile as long as it is out of baby's reach.
My girl loves her mobile. She has a favourite colourful toy and smiles at her toy while the mobile turns. Before we lowered her cot, she was "smart" enough to figure out how to work the mobile herself - she'd use her legs to reach the music selection (that would turn the mobile on)
Also where batteries are concerned, we got the enveloop rechargeable batteries (bad idea as we were charging the batteries every 1/2 days). Someone told us you can get some converter that allows you to use electricity to run the mobile - so those of you who are electrically-inclined, please let us know what you need to do to make this work. Batteries can run out fast when the usage is high!
Get the mobiles that allow you to operate with a remote control. Our baby's mobile has 4 music selections (you will realise that your baby prefers certain music and dislikes others so having several options really helps), 2 light options (at night you can have the stars and moon light effect on and and you can choose to turn off the effect during the day) 2 volume controls (gives you the choice of playing the music at a softer level so baby falls asleep listening to the mobile playing). Bought it at Kiddy Palace for about S$80.00. We bought The First Years brand.
A "must-have" in my opinion as our baby still gets so much pleasure from it - and she's already 8 mths.