I use the marketplace as a jumping off point, it's a place to find new merchants I might not otherwise know about which then leads me to their inworld store. I'm a bit put off at times when I find there's zero inworld presence, it tells me they've essentially "checked out." It may not be so but that's the impression. The MP is also fantastic for sellers I'm familiar with and trust. I can buy their items in minutes without having to go see it first, bing-bang... I'm getting on with SLife.
As for marketplace shopping, I find it a godsend. I've got the patience of a crackhead when it comes to finding stuff in world; rez waits, bad landmarks, wierd layouts, scripty-blinky-buzzy stuff.. guh!!! So on the flip, it's also just as irritating to find people who have no MP presence (which I found lately is alot.)
Personally I have an inworld store and am on the marketplace and I would say the split is about 50-50 but as for commenter who remarked that the MP is ruining SL?... not for me. I've enjoyed and seen more of the world than I ever have in the last 4 years because I'm not spending an eternity sifting.