I love mine, but I remember being a bit underwhelmed at first.
I think MoreSpaceEcho has it right. It's not an instant 'magic' box, but it's a very useful tool, particularly if you're recording rock straight onto hard disc.
The 'warmth' control is basically a high frequency limiter, which is great for avoiding the sort of transient pile-up you can get with a lot of tracks of distorted guitar, aggresive drums, etc. Inline, I tend to use pretty subtle settings, but as a mult, extreme settings can be very effective.
I don't really think much about whether the Fatso sounds exactly "like tape" (which tape? which machine?), since that's not really my goal with it. For adding controllable levels of euphonic distortion, it's great.