This is an older topic and I'm not sure if it is ever referenced but just in case I thought I'd throw in my two cents anyway.
The most effective way to use these files are through a presentation program like MediaShout or ProPresenter. For those of you who are familiar with these programs it runs like a looping movie file broken down into verse, chorus, bridge, extra footage without words in case the worship team goes off into free worship etc. To proceed to the next part of a song you simple click on the section you want and the graphics seemlessly blend (Fade) from one to the other.
Pros: For those of you working with static backgrounds, any motion may be a plus. It can develop an atmosphere of worship and encourage your singers to enter a new level of intimacy with God. It works well with the worship software but don't bother with the basics like Windows Media player or power point etc. Being broken down you can easily flip back and forth between verses and choruses allowing your worship team the freedom to "go with the flow" and not stick to original format of the song. The enclusion of a matching background without words allows you to keep consistancy within the song but without having distracting words on the screen if your worship team likes to "free worhsip". The difference between using Flexx verses a motion background and static words is that usually the words move around as well. If not, there really isn't much difference and the same effect could be used with ProPresenter which allows you to constantly change backgrounds throughout a song if you choose.
Cons: Beside needing specialized software, beware of small text. I noticed that most of the Flexx files (especially the older ones) have text that would be difficult to read, or the flashy background almost overrides and distracts from the words. Watch the demo carefully and imagine you don't know the song and you know have to sing along with it. Text size can be important - especially if you are using older projectors with less lumas (brightness/clarity). Also most are timed to the original song. Which may not be an issue but if your worship team likes to speed or slow things down you may run into problems. For example: we purchased the new Flexx: Revelation. Like it expect the chorus ran faster than what my team was doing so the words were disappearing and changing much faster than we needed them too.
Overall, I think it depends on the individual song as to whether or not I would consider purchasing it. The idea behind it is great. And the few I have purchased, I like. I am hoping they continue to develop this thought, employing the editing technology available, to make the impacting and exciting.