For a travel/walkaround lens you probably want more of a wide angle than 28mm.

The Tamron 18-250/3.5-6.3 (also sold as a Pentax) is apparently rather nicer optically than the Sigma 18-200/3.5-6.3 OS and also a bunch smaller sizes. Of course it's also 50 bucks more and gives up image stabilization but hey everything's a tradeoff I guess.

Review here: http://www.photozone.de/canon-eos/29...-report--revie