I have Opus 1 & 2 (and most other Horizon libraries), and also the Special Edition Extended, and it was worth it for me.
The VI user interface is substantially slicker (read: less time-consuming) than either HALion or Kontakt, and with the free Vienna Ensemble, stretches to many more available instruments than a sampler can offer on a single 64-bit Windows XP PC.
You get some things that aren't in Opus, such as edited highlights of Solo, Chamber and Appassionata strings, Oboe d'Amore, Saxophones, guitars, a piano and an organ. But you also lose some things, such as muted strings and runs, which you might still want to use your sampler for.
I currently use VI for most instruments, but still use Kontakt for things that either aren't in SE, such as the full Solo Strings, Mallets and Vienna Harps libraries that I have, as well as muted strings.
I suggest you look closely at what you use in Opus 1 and 2, see if it is present in SE, and see if the extra instruments and flexibility could be worth it.