Sometimes I use it, sometimes I don't. It really depends upon my traveling party, and whether I want everyone to have charging rights on my CC.
I have it really doesn't save me a LOT of time...a little time, yes, but enough to justify the sole use of online check-in. I do like putting in my cell number and CC myself, rather than going through the front desk personnel. While the vast majority of CM are outstanding and trustworthy, it only takes one bad apple to compromise you CC...and even then, the CM may not even know it. There was a case a few years ago where a bad CM had put a CC skimming device between the keyboard and the terminal, so any CM using the terminal to check people in unknowingly compromised the CC info.