It's definitely fun. Makeb is an amazingly designed planet, with a lot of variety in the natural surroundings, some great architecture and glorious cliffs and you will fall down! (It actually reminds me of Guild Wars' last expansion, Eye of the North. It has a similar feel, esp. in the "jungle" areas of Makeb) I completed the story with my mercenary (and got to 55 in the process) and it's really been great. There are a few lovely scenes and the cutscenes on Makeb seem to have a bit more attention to detail than the vanilla game.
There are also two new questlines, one focused on using binoculars and one on seeker droids, that will see you traveling to pretty much every planet (a lot of which have gotten a new area or two in the process). While it's a fun idea, it's a bit stretched out and gets a little tedious after a while. The quests in between, which demand you to go in lairs and such, are great, though. Some (light) puzzles and some new area designs. Have gotten to the end, which are Heroic +4's and finding people to team up with at this stage seems difficult.
Also did one of the "new" HM Flashpoints. If the others are similar to Athiss, then don't expect major changes to their SM counterparts. I did Athiss with mostly campaign armor (with a few level 66 pieces) and only died at the end because I couldn't evade the fire thingies.