GOG is a great service; their main limitations is that many AAA publishers absolutely refuse to release without DRM, though this is sloooooowly eroding, somewhat. GOG can't go back on their DRM-free promise without a revolt from their audience, but it also limits what big-ticket games they can sign.

I like GOG a lot, they've carved out a nice niche for themselves, but from every developer I've talked to, they're still a fraction of Steam's market share. I don't expect them to take over anytime soon, but I'll always support them and hope they continue to grow.