Anyone used this service before?

http://www.sedo.com/broker/index.php4?tracked=&partnerid=&language=us

Is it reliable / a good idea to go through a "specialist" or just try go after domain owners yourself?

depends on the name and its value
if we are talking about a 5 figure or over domain use a broker
if not, do it yourself

Yes, use Sedo. They are very reliable and know the market. Depends on your budget though.

If you assess well what actually that particular domain's worth for than you should contact direct to the domain owners yourself as I am doubtful about the transparency during negotiation if you're going through a domain broker.