The differences in animal and human responses top drugs are well known.
That doesn't invalidate the testing it just means reducing the risk further. It doesn't need to be reliable to be valuable.
You remember the case of those volunteers in Northwick Park hospital a couple of years back?
http://news.bbc.co.uk/1/hi/england/london/4807 042.stm
Without animal trials you'd see an awful lot more of these types of cases.
An interesting follow up question is whether it's justified to use any animals or whether some should be excluded.
Holland and New Zealand have outlawed invasive testing on Great Apes. I think we should follow suit. I think the level of intelligence and self awareness demonstrated in these animals warrents their exemption.