in my theory, umobile, digi, maxis, celcom got more or less same quality of coverage @ klang valley,
because i m using all 4 lines of them together, every telco might cover 90~100% of the places you used to go, but sadly there are always some places got poor connection for you telco,
my office celcom best umobile maxis sucks, but my house celcom full bar can't online, umobile fast streaming youtube, digi lines are good in shopping malls and high height building which, celcom good at some kampung area which digi maxis umobile always dead.

so better you try the umobile signal out with a RM 8 card @ 7-11 , subscribe a RM 5 1 day 500mb data plan, or just normal use the data @ 0.2/mb to test whether network is good or not for the places you used to be. 

if no problem then u might look for me to port your current number or find a vip number for umobile with me