I'm currently considering three lenses for this duty, but from those I'd ideally like only one, or two, at the very most. In researching these lenses, some consider them either perfect for the job, or mediocre, or terrible. (How often does one get any real definitive answers for questions like this anyway.) These are Canon 18-55IS, Sigma 18-135(non-IS, the older model), or Canon 28-135IS. I have my own thoughts on this decision, but would like your feedback first. I define "walkaround lenses" as ONE lens that can be utilized for MOST situations where the actual subjects may not be fully known. Maybe something like for a tour, or shore excursion from a cruise ship....where you really don't know what you'll see until it's right there in front of you, AND traveling light is beneficail. Thanks.