Boom Town isn't *that* bad. It definitely goes a long way in refining the Slitheen a bit, and is a far better depiction of the alien race as much more complex than their original, more simplified and cartoonish appearance.

I like how these aliens are humanized a bit even while they are still not necessarily good. It challenges the Doctor a bit. Boom Town is also almost invaluable as one of only a small handful of stories with the Doctor, Jack, and Rose. I wish this period were a little long in the original series, because the dynamic of this trio is really something. Throw in Mickey and we have a pretty great full blown TARDIS crew in this episode, and I think all four characters are used fairly well in this regard. They did a good job slowly introducing us to these characters and letting us get invested in each of them, to the extent that even a more mediocre story like this is heavily elevated by our characters.