I have recent hit level 40 on my bodyguard merc and have been enjoying it very much. With all my talent points in the bodyguard tree I am probably lacking a little bit of exta dps while questing but using Gualt or Torian works out great.
I have (solo) healed every flashpoint up through Foundry with no issues at all. It really is all about heat management. I have played a few other MMOs (and a LOT of WoW) and the heat system is unlike any resource I have used to heal with; and DOES TAKE TIME AND PRACTICE to get used to. But once you get the hang of heat management and get abilities like Supercharged Gas and Kolto Shell healing as a bodyguard is very engaging and fun (my opinion of course). *NOTE: this is the only character I have played thus far so I can't really comment on comparisons to scorscer or operative healing.
Bodyguard DPS
A friend I play with a lot is an Imperial Agent Operative healer and when running FPs together I usually let him main heal (as I am a few levels higher) and help out if needed (which is usually unessesary). Although it would be nice to have a meter of some kind to support this, but it seems that bodyguards can do a fair amount of dps in flashpoints if needed, mainly though the use of Supercharged Gas and Power Shot/Unload. *NOTE: In no way am I recommending bodyguard as a straight dps spec, mearly stating that bodyguard is not completely useless as a dps.
Last night a few guildies and a few PUGs (8man group) killed the Ancient One (a Taris world boss) with myself and my operative friend healing and niether of us had any problems keeping the group healed.
If you are looking for a WoW comparison I would say bodyguard is most similar to a holy paladin, in that your main heals are cast (activation) time abilities forcing you to remain stationary. But that is as far as I will go with similarities since as stated earlier the heat system is quite unlike mana. *EDIT: also similar in that a bodyguard, while capable of solo healing smaller groups (i.e. flashpoints) are very stong single target healers.
I highly recommend playing a bodyguard for anyone looking for an unique and engaging healer expierience in this wonderful game. Cheers!