Spent about 10 days there in August, for a friends wedding, a few years ago. It was very hot, but not unbearable.
Quite a bit group of us went and we all loved the place - even bearing in mind that some of the group couldn't be classed as "country music" fans. Because, every where you turn it's country/gospel/bluegrass music. The pubs and clubs were great - really friendly and music music music.
We visit the museums (which are fantastic), the Grand Old Opry. Hired a car one day and drove to Memphis & Gracelands (well worth a visit). Would have liked to stay over for a night or 2 in Memphis.
The wedding was in the Grand Station Hotel in Nashville, which is absolutely amazing. Go for it and make sure to record your own cd while you're there!!