I definitely don't want to be a summoner....frankly, I don't even know why that option is in the game. None of the creatures you can summon are powerful enough to useful past like level 15 so trying to make a summoner build is a complete waste of time in my opinion.
I fully disagree with that, the mage summoner build is one of the strongest mage builds you can have. A level 18 iron golem is a terrifying creature to be able to summon at will. It has good resistances and high armor. As a competent tank and decent damage dealer it made end game areas like the Citadel and Ark a breeze to go through (except the radiated parts). It can be combined with Hirge to heal it and be absolutely unstoppable. And even if your summon dies you can quickly bring in a new one to finish the job. And since you are able to summon it inside the Ark, you can have a tank even when you don't have a companion with you to protect you.
And even besides the really powerful iron golem, the rest of the summons are a big help as an extra source of health and damage, or as distraction while trying to escape a horde of monsters. The sparkling was great in the early levels, but he's not as competent as the golems. The fire golem helped me clear the Troll area at a very early level. The only summon I didn't use was the Ice golem since the Fire golem was more useful and afterwards I switched to an Iron golem.