Summoner like the Blackmage is a caster DPS role that relies on pet input. Your DPS mainly comes in from DoTs and ensuring that they are all up. For single player content I believe that it is one of the best out there as you are loaded with your own healing spell, MP regeneration (BLM has to balance Astral Fire/ Umbral Ice procs for their MP regen, and ours is just one button) and tank pet. The only thing we don't really have to start off with is a good AOE and burst DPS.

Plus when you are able to do the job quests you get to have primal's floating around and nor just carbuncles. Arcanist is also currently the only class that can have two jobs.

But I am a bit biased as SMN is my main. It's up to you on how you wish to play and what cracks that cheeky grin on your face when you down content.