Replying with the best of my knowledge. Note that I am recommending you based on the fact that I know them (1 of the staff is my Pri School classmate) and hang out there long enough to to know they are safe and reputable enough to have customer retention and referrals.
For what it's worth, they helped me repair laptop for several occasions (in different years hor, just in case you suspect they never do their jobs well which made the laptop problematic)
Who: PC Clinic, Sim Lim Square #04-20. Boss' name is Kenneth
How was service: No-nonsense, I would say. Just be normal, don't ask stupid or unrealistic questions. But got interns there, so ask older-looking staff to get satisfactory answers. Same goes for everywhere lah, even you yourself also will be pissed if I ask you to do things as if you are a god.
How long: Since it is battery replacement and not diagnostic, maybe a day. Also have to depend on their amount of repair requests. If you don't sound pushy and maybe have valid reasons, they might be able to settle for you within the day. But again, have to depend on their workload.
Cost: Dunno. I can ask my friend if you want me to.
Reformat: No need. Only official service centers do that with excuse of needing to help you update the firmware. But the main reason is for you to sign an acknowledgement that you are aware that data will be erased, because they scared the staff abuse your secretive photos and videos and spread them around. Big corporate companies cannot afford such scandals.