SL is (was) over-hyped and the backend is cuddy. Nice front-end tho!
For all LL's harping about OSS, the voice client is not available in the Linux client as their tech supplier doesn't support all their client platforms. Being a Linux user is therefore the ultimate excuse for not participating in the voice option ;-)
My own interest in SL is largely in building 3D stuff for the amusement of myself and others - I basicly treat it as a virtual Lego-technic/Meccano set. It is much more like a free-form play toy than a 'game' with an explicit objective, hence people who are used to living by canned objectives (kill the monster get the gold in VR, or, do the job get the house/car/HDTV in RL) tend to be disappointed ;-)
While I can think of a number of VR systems with vastly superior back-ends, SL's UI is the only one I have encountered that doesn't actually require you to be a programmer or 3D expert of some sort to make your own in-world stuff. That is honestly the only reason I use it.
As for the busines world, good on LL for milking lots of development capital from big-business 'managers' who should (but never do) know better than to be sucked in by novelty and promises. OTOH, advertising write-offs might be a very carefully thought-out move on their behalf, so I probably shouldn't assume stupidity (though on past record, it is a pretty safe assumption with most big-business). That isn't to say that SL has no business use, but it isn't the 'new-era' by a long shot. Just as in the long-distant past, private companies could pay for brand placement in Lego sets (I assume Shell paid for that), there is a case for some sort of presence in SL, but probably no more than there is for a presence in a PlayStation game road-side/field-side billboard. A corporate presence in SL is not much use unless the presence is more than a huge 3-D billboard. If I want to see a company's products, their web site is a more usable alternative to wandering a virtual showroom.
As far as the market slump goes, in my own recent experience you are spot-on. Over the past 3 weeks I have noticed a serious flattening in the in-world marketplace for my sales of some of the stuff I made that others seem prepared to pay for (my aim is to cover my online costs, not to actually turn a proffit for RL, and in that I have been more-or-less successful so-far).
SL is really free-play. A small number of people have turned their play into a profession, but most just play for the sake of play and some are like my brother's garage band who are happy to make enough at pub gigs to cover the costs of petrol and to get new instruments occasionally. Don't be expecting to give up your day job for a virtual one any time soon!