Being for the most part a personal decision by the blogger concerned, the 'having fun' part is easiest to accomplish (unless you’re a natural curmudgeon, like a certain old gran of my acquaintance).
Oddly enough, the most obvious of the three, selling books, strikes me as being the hardest to achieve. Sure, some people will make an impulse purchase based on the cover pic and blurb that goes along with a guest post, but it's by no means guaranteed.
It's much easier to gauge the benefits to be had from putting yourself in front of folks who don’t yet know you exist. Assuming you write something of interest, I can certainly see how a percentage of new readers might check out your own blog or website and subsequently stick around.
So what do you write about? Do you go for the interesting subject or a promotion-related post? Do folks interview you or the characters in your book? I'm leaning towards a combination of all these things. Only time will tell which ones were most effective.
How about you?
What’s your take on blog tours?
ETA: As you may have already guessed, I’m in the process of setting up a blog tour to help promote next month’s paperback release of Fur-Face. If you’d like to have me as a guest, please let me know: firstname.lastname@example.org.