I used it to develop a Birds in Flight slideshow for a St Louis Camera Club presentation last year and it does allow customization, lots of transitions and music fade in and fade out (you will have to do a time line adjustment for each image but it isn't that difficult-- it shows you the duration on the image 'frame' as well as the type of transition)
They have a 15 Day free trial
check yur pm
I think Roxio and Nero Burning also have slideshow programs but I don't know about the customization feature
There is probably some freeware out there if you do the Google,Bing or Yahoo search engine 2 step