I purchased the kenko extension tube set last year and have been really happy with them despite:ETTR or manual exposure.
more ISO noise
more light required to get proper exposureYup! Light 101 - the intensity of light is inversely proportional to the square of the distance... So expect that to happen.
more sensor dustDon't know about that...
losing infinity focusYup! So only works for macro work...
You might also want to look into a macro focusing slider.+10000! You may want one that moves along two axis. If you ask neuro he'd say get the RRS one...
Here are a few pics i took with the kenko tubes and the 5D II + canon 100mm 2.8 USM macro lens.Lovely!
1. Japanese Hawk Moth Caterpillar (スズメガ)
2. Japanese Giant Killer Hornet (オオスズメバチ)