Yesterday i did my first muni/cross-country on a 28 inch.
It was great the speed is awsome you fly over the bumps with the big wheel.
Going up hill was just as easy the big wheel and speeds made it easy.
I had 150mm cranks and my knee was not hurting that much as with 110mm. 
The track had alot of mud and i had normal road tire that was fun slipping al over the place.
MUni is good.
What size wheel were you on before?
Normal i ride an onza with a 24x3 tire
I also ride a 24X3 tire, and have also been considering buying a 28 tire. Although, I recently rode my friends 36 Coker, and he might sell it to me (frame and all).
Do you find that you are able to maitain control when going slow?
i think if your in the buying field then look at the KH 29er.
unless you want to go really fast
I still have control while going slow, the 150mm cranks made that possible.
With 110mm it’s alot harder.
well I also use a 29er with a BigApple on trails
after many months on 150 I switched back to 125 : with sufficient training those proved maneuvrable after all (except on very technical terrain)
but the most surprising thing is when going up (slopes are my nightmare I still do not manage to climb steep slopes) though 150 give more torque I finally found that I would go further on 125 just because on 125 the phase where you are out of control is shorter: strange… are other people experimenting the same thing?
this said the big apple is not perfect for trails: slime, gravels and so on lead to UPDs
A 28 is very capable cross country. It smooths out the bumps which allows you to attack hils with more speed, and you are less likely to stall. Therefore, you can often get away with shorter cranks than on a smaller wheel. Strange but true.
I have done a lot of cross country on a Coker, with some quite steep inclines and declines. It is very capable off road.
Where you do lose out with the big wheel is in slow fiddly sections. The big wheel is less responsive to sudden changes of pedal pressure (fore and aft balance) and to steering input.
You therefore get this strange situation where a Coker or 28 will fly over rough ground and up hills that would be hard work on a typical 26, but once you lose momentum, the “flying machine” becomes a lead balloon.
On a 26, you are a jockey; on a Coker, you are a mahout.
An crank size? I bought my 28 with a standard 32 (or 35?0 mm tyre and 110 mm cranks. I did a lot of serious cross country on it. Only the hills were a problem. I now have the 28mm almost slick tyre and 110 mm cranks. Only the traction is a problem off road.
Whatever you ride will suit some styles and route. The variables are wheel size, tyre section and crank length. 3 variables - one compromise.
Get a 31", there better.