There’s an article about VSTO at SD Times. One of the things they point out is that VSTO is the first Microsoft developer product that really enforces a strong security policy, and that this is the way of the future (think Longhorn). The author also likes the network-based deployment model, although I would caution against using URI evidence if you can go with something stronger (like x.509).
The article mentions that VSTO retails for $499, but fails to mention that this prices includes VB, SQL Developer, and the Access Runtime. There is also a $199 upgrade available if you already own VS or an older Office development product. See the “How to buy” page for more info.