Paul, we had considered the DIY route ourselves, but the advantages of using Equios seemed to outweigh this, mainly in the following areas (for us of course - might be different for you). I'm sure it's possible to setup/write your own code to do all of the following but its definitely not a trivial task!
1. The ability to show separations - especially where spot colour is concerned - I'm not sure you could get viewing software off the peg that would do this.
2. The ability to preview the artwork with and without the bleeds showing.
3. The ability to automate the whole process of ripping, trapping and sending an automatic notification to a client.
4. The ability for the customer to approve a job which then automatically goes on to the next stage.
5. The ability of a customer who has correction pages to show both original and correction pages in a window side by side with the differences between the two pages highlighted.
6. The ability to let a customer login and Create/Upload their own files and have a ripped proof send directly back to them.
7. Some of our customers act as administrators for their own proofs - they can allocate who in their department gets proofs, approval rights etc.
When we first started with online proofing we didn't let customers create/proof their own jobs, but we now have a number of clients who really like this idea - it's a good way of connecting with and keeping some customers. The downside (and I guess its a good thing) is that we sometimes come to work with a number of unexpected jobs waiting to go to plate having been uploaded, proofed and corrected without us being involved. All we have to do is get the thing printed at the usual breakneck speeds expected nowadays
PS if for some reason you can't get a demo from Screen PM me and I will send you a link to an online proof on our system for you to look at.
PPS. There is an app available (its really a web browser) for Equios Online on the Ipad, and I believe that some of the proofing options such as Page flip don't require java as they are html5 based - indeed I seem to recollect during discussions with Screen that the Equios Online proofing was going down this route - I have heard that Equios 2 is due shortly so I wonder if the proofing is also due for an update.