It's very useful for avoiding queues and fiddling with exact change, especially now that KTM Komuter and the Monorail take it also. (AFAIK the KLIA Ekspres regrettably still does not, despite the T'n'G site's claim to the contrary -- somebody please correct me if it's finally working!)

The problem is finding a place to apply and top it up, as only a few of the larger stations do this and even then only at certain times, even if the general ticketing counter is open. I've found the best place to be the dedicated T'n'G counter at KL Sentral, open 10 AM-8 PM daily (closed Sun). You'll need to pay a RM 10 fee and fill out a silly application form asking for name, address and all sorts of crap that nobody pays attention to -- for example, my current card is signed out to Mr. Mickey Mouse of Disneyland -- but once you've got it you're good to go. Stays valid for 12 months after use and can be reactivated even after that for RM 5.