OK I did a bit of A/B comparisons with a few of the other things mentioned (I don't have the full Vienna or any of the EWSL stuff, so can't comment on those) and here are my impressions:
1) DSF pretty much blows the SI strings out of the water. Not that they're useless, just... very limited. And not very 'deep' or realistic in terms of soundstage, etc. They sound pretty plastic-y in comparison to just about anything. YMMV, but that's what I'm hearing.
2) DSF is probably a bit better, and certainly a lot more flexible and varied, than the Garritan Pocket Orchestra that I think is included stock with Dimension. The Garritan ones are quite OK, but there's not a lot of variety.
3) DSF is not as flexible or complete in terms of articulation as the strings in Kontakt (which are a kind of basic Vienna subset, or so I am given to understand) but they're probably easier to use if you just want to throw up a quick big ensemble quickly or are looking for a big cinematic sound.
4) unfortunately, there are some pretty glaring tuning problems here and there in DSF, which makes some of what's there unusable (for me, at least). Again, a fair bit of it seems like filler to me to make it seem like a bigger library. I would be pretty peeved if I'd paid full price for it.
5) However, there are some really nice-sounding sections and ensembles in there (the small sections are a nice touch), and you could do a lot with it if that's all you had. So again, if you're short on string sounds this gets you a decent palette of usable stuff, along with some annoying filler that, who knows, other people might love, for a pretty good price (free).
and that's my two euro-cents.