I'm one of the ones who's raved about the Intensity vest (also the larger Synergy).
There have been some durability issues with the bladders and parts, but so far Nathan and/or Hydrapak have been very responsive. This is actually a small ding - the latest part that broke was an elbow connector and I really considered it normal wear and tear. I'd have been much happier to buy a new one for $5 or $10 rather than try to rely on the warranty department, but they didn't have them for sale.
Another question I have is that they've redesigned their 2 liter bladders with a hard slider across the top. I can't imagine this would be comfortable on most women's narrower shoulderblades. That's a speculation and not an experience, but you might check which bladder the new Intensity packs are shipping with and search other forums for people's experiences with the newer bladders.
Speed comes from what you put behind you. - Judi Ketteler