I hate changing lenses out of doors. Dust, dirt, water, pollen - it's all evil to a camera, and it all wants desperately to get in. So I bought a "superzoom" class walkaround lens, and I use it for just about everything, from family gatherings to vacation shooting. I've even used it to shoot a couple of weddings (as second shooter, of course - I'd want Canon L-series glass if I were primary shooter on a wedding).
When last I ventured out west and visited Grand Canyon NP, Red Rock Canyon NCA, and Nevada's Valley of Fire SP, my superzoom was a Sigma 18-125. I found myself chanting a mantra: "The wide ain't wide enough and the zoom don't zoom enough!" It was also, as a cheaper lens, a bit soft and had some barrel distortion.
My current lens is a Canon EF-S 18-200 IS (F3.5-5.6). This is a wonderful lens on my Canon 50D, and is well worth the $600 price tag. NOTE: $600 is a HUGE, HUGE, TITANIC amount of money to me, so I don't say that lightly. This is the most expensive lens I've ever bought, by nearly a factor of 3.
Using a superzoom as a walkaround lens has pros and cons:
PROS:
* Fewer missed shots due to changing lenses
* Less chance of crud, mud, or flood getting into your camera because you don't change lenses outdoors as often (or in my case, ever)
* Less weight to carry, since you have One Lens to Rule Them All
* Fewer filters to carry, since you only need them for one lens
(Note: Those last two are pretty important when you're hiking in wilderness areas like Yellowstone and Grand Teton)
CONS:
* Superzooms aren't L-glass; they're not as sharp or as bright, and tend to have more vignetting, barrel distortion, and chromatic abboration than primes or L-lenses
* Superzooms have variable aperture; they're not available as primes, thus are not as good in low light
* The EF-S 18-200 is not a USM lens (god knows why)
I'll leave you with this final thought - I have an EF-S 55-250 IS. I find that my EF-S 18-200 is superior to it in every way except the long focal length, but I find that the difference between 200mm and 250mm is negligible.