The only legendary summoner I have is the Llama. It's a really powerful card in certain situations. When you have a high mana cap and the high mana cost of 7 for this summoner isn't such an issue, it's a no-brainer.
There's another way it can be useful, though, even in lower mana battles. If you plan out the Last Stand thing right, your last card can just wipe out everything in its path.
The last win took me a while. I was only playing sporadically, and I even accidentally fled the battlefield a few times because I was also working on making breakfast.
As you can see, the rewards weren't all that great, but I'm only getting three rewards. The odds of getting something noteworthy and awesome in only three rewards is pretty low. It's not impossible, but I definitely don't count on it ever happening.