I like my photos taken with the cheap Canon 50/1.8 prime lens (like these) more than the ones taken with my Canon 24-85 zoom lens (like these). Prime lenses really are sharper and have clearer colours than zoom lenses. But zooms are convenient, particularly for walking around.
The usual suggestion for an excellent all-around zoom is the Canon 24-70 f2.8L. But it's very expensive and at 950 grams, too heavy. An alternative is the Tamron 28-75 f2.8. It's got the same basic specs as the Canon, but a third the price and half the weight. Folks seem to love this lens.
So I bought the Tamron and did a quick comparison with my Canon 24-85. The photos are ISO 100, f/8, shot on a tripod, JPEG straight from the camera. In the sample above two things stand out. The colour on the head is better in the Canon, but I think that's just luck of the clouds. And the reflection in the candlestick on the left is so much sharper in the Tamron! I think that's for real. We'll see.
This comparison is a bit weak; it's hard to really measure lenses. But I've got an example that convinces me this lens will work out. See the Flickr set for all of the photos.