Rootkits are the ultimate in stealthy malware, burying themselves so deep into your system that they're often very hard to spot.
Kaspersky's TDSSKiller as the name suggests, is designed to target a few, specific threats (TDSS, Sinowal, Whistler, Phanta, Trup, Stoned). These can be very dangerous, though, so having another way to pick them up isn't going to hurt -- and the program can also detect hidden services, forged files, MBR changes and other suspicious signs that could indicate infection by a brand new rootkit.
TDSSKiller is very easy to use -- just launch the program, click "Start Scan" and wait. You won't even be doing that for long, because TDSSKiller checks only the most likely areas of infection, your services and drivers, and so returned its verdict in only 14 seconds.
Certainly TDSSKiller isn't going to replace specialist antirootkit tools like GMER any time soon. It proved good enough to spot a malware. The program even supports command line options that might allow you to script its actions, running it at boot time or on other system events.