Gallup says only 38% say his speech was excellent or good. In comparison, 47% said the same about John McCain's speech in 2008. 16% rated Romney's speech as poor or terrible, which is also the higest number since 1996.
40% said Romney's speech made them more likely to vote for him, while 38% said they were less likely after the address. The 2% difference is also the lowest hey've recorded since 1984.
No matter how you parse these numbers, it's clear this is why Romney is likely not enjoying a large post-convention bounce in the polls.