That's a very legitimate question. And to be honest, I've never heard of one work and have ony heard of them not being successful. Maybe only people who experience failures are vocal about it, or maybe none of them work.
Anyone ever seen one work successfully?
I've done it enough that sometimes I do it ghetto myself... Remember the old-school record players?? There is a disk/platter/record that spins, an arm that reads/writes and a little printed circuit board (PCB) that controls the whole fiasco.
I start by swapping the printed circuit boards and holding my breath - take the OLD PCB and put it on a known-good drive. (exact firmware, buy used, eBay) If the PCB fails on the known good drive you are LUCKY... put the good PCB on the "bad drive" and it'll most probably work.
If the PCB works on the "known good" drive then pardon my french but you are F'd... open up the case (carefully) and put the platters from the bad drive into the "known good" case/drive... fire it up and pray...
Sending it out is pretty much a black & white process - either they get your data or it's too far gone... For the cost of data recovery on a drive you can have 10TB backed up in a RAID5 or RAID6 DROBO (Do you know they sell 6TB drives these days??)...
Do not EVER put yourself in a position to need to send out a drive - it'll take MONTHS, cost an arm and a leg and won't always work... So that's that...
EDIT: To follow up - you then use a command line and block-for-block copy the disk over to a completely good/new/known good drive/raid array (something redundant) and then try to recover/restore once you can make an archive/master copy and a "working" copy... Don't just swap stuff over and try to fire up Windows and work with it...
EDIT #2: For reference... I pay more attention to dirt/dust when I rebuild the fuel injectors on my race car than I do when I swap over platters on a HDD... but to get comfortable with the process I bought a lot of 20x 40GB HDD's on eBay and destroyed just about all of them teaching myself - don't just assume you can do it.