The strings are... umm... OK, in a word. They are better than the demo makes them sound, but that's not saying much. There are a bunch of total throwaway filler patches with not particularly great or imaginative sounding effects put on them, which is about as useful as a chocolate teapot; I am personally quite able to apply a vanilla phaser to my string sound if I think it would sound good, and I suspect that I might not be alone. The straight up ensemble and section patches are decent, but as others have said nothing to make the Vienna folks tremble (heh, I'll be living in Vienna this winter for 5 months!). I haven't A/B'd them against the SI ones or, for that matter, the Vienna subset that comes with Kontakt, which is what I tend to use if I'm going for realism (if not, I'm as likely to use Mellotron or retro synth strings as anything else). My gut feeling is that the DSF ones are kind of somewhere in between the two.
If you're really on the fence about whether to upgrade at all, I wouldn't let the bonus sway you unless you are really hard up for string sounds. However, if you feel like you'll probably upgrade at some point anyway (it's hard for me to see why anyone wouldn't unless you're changing platforms) I doubt the price will ever go down, and this is a little perk for getting in on the ground floor. There will probably be other special offers later, mind you - other freebies, whatever, but I personally can't see the price getting any lower. Then again, I've been wrong before!