I don't have experiences with Siebel to share, but last Wednesday we installed a SH520 next to our regional Sharepoint infrastructure (it's in Singapore, and I'm in Australia with link latency of ~110ms) and the improvement has been substantial. This is a real bonus for us because the pre-optimisation performance was somewhat hampering uptake of the platform. We also heavily use a 3rd party workflow app alongside Sharepoint called WorkFlowGen, which is built around Acrobat forms which has also seen a very significant improvement. This is mainly because the PDF forms are 150k-300k each, but of course the form contents are typically retrieved every time ... something we can now avoid thanks to the Steelheads.
So, after three business days of production usage we've got 70% reduction in HTTP and I expect that will further improve as more sits on the cache.
Using a typical 4MB zip file from a Sharepoint teamsite as a tester, post-install there it was 5½ times faster when the zip was completely cold, and 126 times faster when it was warm.
You could certainly say I'm very pleased, as are the users!

We also have Microsoft SMS infrastructure in this same Singapore data-centre, and this has seen a 20% reduction in traffic - not as significant, but this will typically be compressed SMS package replication - so I wouldn't think there's a lot of repetition in the data stream. Still 20% is a good result regardless since the SMS packages are typically quite large.
(I'm also trialling Riverbed Mobile at the moment and plan to do some Sharepoint testing on that too sometime soon.)