Honestly, I've been considering this as well, Red Tie. I've gone back and forth, and really, most client don't really care to have the slide show in their package (now that is MY experience... I'm sure it differs)... in fact, I have it my larger packages, and it's the first thing they take or swap out when trying to customize the package.
Anyways, I've decided to start using them more now, mainly to embed it onto my FB page, and let the client really fall in love with it and want to purchase the DVD ala carte.
Everyone is going to have their own preference and reasons why the use what they use, so take everything into account when you choose and find what fits your needs the best.
I've found that Animoto
offers a quality product with literally no work involved. ProShow is more work than I have time for. You can have web slideshows that give great quality and dvds that have awesome quality as well. they're animated and you can add video clips to your slide shows, as well. But my fav part about it? All you have to do is upload it to the site and they build the show for you. Every single one is different... and the animation (transition of the photos) is really cool and entertaining. I almost feel like I am watching an actual video. For this company, you pay per year, and you have unlimited slideshows that you can make.
I've never used showit, so I don't know much about it. But I have used ProShow and I wasn't too happy with the time it took to create an awesome slideshow.