The one thing that irritates me about these things is how the spins are intentionally slowed down to make each spin last a proportional amount of time relative to how much time you are alloted and how many coins you bet on each spin. For example, the Tomb Raider tournament gives you 5000 coins and 5 minutes, at 75 coins a spin. So 5000 / 75 = 67 spins to play out in 300 seconds, and 300 seconds divided by 67 spins = approximately 4.5 seconds per spin. If you play the regular slot, a spin normally lasts no more than a second or in the case of Thunderstruck, more like 1/3rd of a second. Plus the tournaments don't recalculate the spin time if you happen to get the free spins - each remaining "paid" spin should in theory be faster, but instead you just run out of time with coins remaining (probably in the hopes that you'll rebuy).
I was convinced that they were "rigged" when they first came out, because 3 different tournaments in a row I happened to get the free spins with less than 10 seconds to go or on the very last spin. My thought was that they wanted you to rebuy in order to continue. I have changed my position on this now, but because they were new at the time I'm sure you could understand my suspicion.
That being said, one night I won the top prize in a $200 Thunderstruck tournament (a $60 bonus) because I happened to get 5 hammers (4 + kicker) during the free spins = 22,500 coins plus a bunch of other little wins along the way. I also won a $25 bonus from one of the larger invitation-only tournaments that happened a couple months ago (one of the ones where you get 10,000 coins and either 10 minutes or unlimited time, I don't remember).
So... I think they're a welcome addition to the MG suite but I would like to see a few more [of my] favorites added too, like Spring Break (despite the fact that it's the same as Thunderstruck), Harveys, Loaded, or What on Earth.