The Mystic form of a Saiyan was only shown in the series once. It was also shown in a movie or two. The only Saiyan ever to reach this (show or movie), was Gohan. In the series, it is thought no hope is left for defeated Buu. But after Gohan breaks the Z Sword, the Elder Kai is released. He agrees to train Gohan, if he could get a kiss from Bulma.
So, after a long warm up ritual, then hours and hours of just sitting there, Gohan's Mystic powers were released. Although not Super Saiyan, his powers were far beyond even that of Goku's Super Saiyan 3 powers. Super Buu was no match.
The Mystic Saiyan form is the strongest Gohan ever becomes. Since he is a half Saiyan, he can't go further than Super Saiyan 2. So this gave him the chance to show his real hidden powers. It is a power up that lasts only 24 hours, but gives the warrior almost unlimited strengths, or so it was thought until Gohan had to fight Gotenks Buu.
When in this form, the Saiyan has their normal colored hair, except it looks a bit more spiker, as if it were Super Saiyan. There eyes too, keep their color, but have more a glazed, shinier look. With the incredible increase in power, they are also much more confident. This is proved when Piccolo is trying to figure out what's different about Gohan. He says that he is a totally new person - much more confident. Although I've never heard of confidence making a new person, you get the picture.
I think that if Gohan could have kept this form throughout Dragonball GT, enemies such as Bebi and the Evil Shenrons would have been defeated much more easily. Then again, Gohan doesn't keep the powers..so..they are not defeated much more easily.
To end this quick, Mystic Saiyan is the strongest form of a Mixed Bloox Saiyan. If it weren't for Super Saiyan 4, it would be the strongest form of a Saiyan, period...