There was another thread where I provided an update:
http://festoolownersgroup.com/other-tools-accessories/hammer-sanding-attachment/msg406037/#msg406037
I haven't used the attachment since in a serious way - I just tried setting it up, briefly trying it, and putting the blade back as an experiment. It was a bit easier this time as I had left the various components of the sanding attachment connected together to save time. I think my original view (as expressed in the above link) is largely the same.
I would buy a Jet JBOS-5 instead (if I could find the space for it). If space was limited I would probably keep the sanding attachment and give it a bit longer to prove itself.
I think that the sort of woodworking I do (and I don't do that much due to lack of time) does not require this type of sanding very often hence my not trying it out much.