I think your eight year old will like it, I don't know about your 5 year old...depends on if he/she likes playing N64, Playstation 2, etc. My DD's are not into video games (one 4 1/2 and the other 8), and I would still not pay the extra money for them to go. My DS who is 12 says it is his favorite spot in WDW.
I agree about the noise; it gives me a headache around 2 hours into it. It is also dark and I find difficult to get around with the five stories; I got lost a lot. There is a great Cheesecake Factory on the fourth floor that has the most wonderful cheesecake!
[I am glad my DS is now old enough on our next trip to go solo without parents, with some other boys his age for the afternoon. He will have more fun without a mom who can't play video games at all and be with kids his age who appreciate this place more than mom does!!!]