I once used Opodo UK to buy a refundable business-class ticket, then later decided I was unable to travel.

I began the process of cancelling that ticket and seeking a refund on 13 Sep 2013, contacting them via phone or e-mail about once a week. They paid me back most of the money fairly quickly. But an agent early in the refund process insisted that the "YR" surcharge component of a refundable TK fare was nonrefundable, which is not consistent with the fare rules or the airline's contract of carriage or, well, reality. After a few months of weekly back-and-forth I looped in The Guardian's consumer advice columnist on 20 Feb 2014. After a few more months of weekly back-and-forth I and finally actually got the correct amount refunded on 27 May 2014. They also included a 25 GBP goodwill certificate for my troubles.

Overall my impression was that the company is capable of issuing tickets which autoprice, either via the web or via agents, and some of their employees are capable of some basic GDS interactions. If you don't plan to do anything other than book and fly the ticket you should have no problems. My personal experience would make me wary of using them to change or cancel itineraries in accordance with fare rules.