I rate Spong very highly. He is probably my favourite kickboxer and he seems like a really nice guy despite his vicious style and menacing look.
There has been increased exposure of him to MMA fans since he linked up with King Mo and then became Rashad's training partner/corner man. I would check out his fight with Overeem during the K-1 2010 GP. Overeem is of course regarded as one of the best strikers in MMA but Spong held his own despite being a much smaller man and he had a harder fight than Overeem earlier in the evening.
As far has his grappling, I haven't heard much about the quality of it but he has certainly surrounded himself with good coaches and training partners in Florida. He is relatively young but it will be a struggle for him to pick up wrestling to a high enough standard to deal with the likes of Phil Davis and Jon Jones (I would add Evans to that but I doubt they would fight each other).
A few more wins outside of the UFC and I expect him to get the call. If he is able to get his grappling to even mediocre level, there are plenty of exciting fights for him in the UFC.
Always appreciating the fine workings of Sakara, Almeida, Gonzaga