I hunted last Feb and it's definitely not the easy road hunt like it used to be. That said, if you have some western hunting experience, have optics and know how to use them, you don't need a guide. Just be sure to do your homework on trophy judging.
In my opinion, the lower success rates have more to with the animals becoming more educated than decreasing numbers. However, we are still talking about 70%+ success rates. That's pretty high for a 2 day hunt. If you are used to hunting elk and deer on public land out west, you should not need a guide for the hunt. If you are really concerned with the type of trophy you want, then you might want to hire a guide.
I have a couple of sons that have hunted the African planes game and they tell me US hunters have some what of a problem as most of the African game have harts that are quite a bit lower in the cavity.