There is no good replacement app for iDVD when it comes to ease of use, flexibilithy and professional looking results.
You'll have to purchase an iLife disc and should be aware that the iLife 11 disc only provides iDVD 5-7 themes. The Software Update no longer installs the earlier themes nor do any of the iDVD 7 updaters available from the Apple Downloads website contain them.
Currently the only sure fire way to get all themes is to start with the iLife 09 disc:
This shows the iDVD contents in the iLife 09 disc via Pacifist:
You then can upgrade from iDVD 7.0.3 to iDVD 7.1.2 via the updaters at the Apple Downloads webpage.