I currently use my 28-55mm 4-5.6 Canon lens as my prime lens on my EOS 40D, but I wish I had a Canon 50mm 1.4, I used to have a 50mm f1.4 on my Pentax and loved it.
Or, go for the EF 50mm f/1.4 USM for about $400US. It's a very sharp lens, and I'm finding it to be very useful for everything from landscapes to flower close-ups (not macro). It's an excellent investment, one that will migrate gracefully to your next EOS body upgrade.
By the way, just for info, most of us think of a "prime" lens as a fixed focal-length lens, not a zoom, so I'm sure you meant "primary" for your 28-55mm, right?
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