Backpackers heading into the Rockies are advised to purify their water before drinking to avoid infection by bacteria and parasites such as giardia. Personally I have my doubts about the likelihood of getting sick from contaminated water in the Rockies (have you ever actually seen a study on this?), but have always used a filter "just in case". The filter we've used for years is the MSR MiniWorks EX filter, a very popular "lightweight" filter that screws onto a standard nalgene bottle. Filtering water to get a drink involved getting the filter from the pack, sitting down by an often cold stream, screwing the filter onto the nalgene bottle, pumping for a few minutes to fill the bottle while holding the cold filter in your colder hand, then unscrewing the filter and repacking it. We'd often get mosquito-bitten too as both our hands were tied up pumping and unavailable to swat the buggers. The entire process was cumbersome and frustrating when all we wanted was to get a drink. Not only that, the filter weighs 439 g when dry and the nalgene another 172 g, meaning I was lugging around at least 611 g (~1.5 lbs). For 6 years it annoyed the heck out of me.
The MSR Miniworks and naglene bottle setup we used for many years. So this past winter I (once again) tried to find a better water purification option. I didn't waste time considering any annoying pump-based filter, nor any of the dead-simple gravity filters as these are meant for use in-camp, not on the trail. Chemical treatments seemed like a good bet as they're the lightest and easiest option, but to treat water at a typical Rockies temperature - 4 degrees or so - would require several hours, a ridiculously long time to wait when you're thirsty! Next up were UV-light treatments, but they are only effective in crystal-clear water and thus aren't likely suitable for the often turbid water of the Rockies. They also use both batteries and a lightbulb, two things I don't particularly trust to always work when I need them too.
One somewhat newer class of filters looked particularly interesting: Those that can filter on-the-fly as you sip water through them. No pumping, unpacking, or waiting required! They're just as or more effective against bacteria, cysts, and protozoa as the other types of filters too. Some contain a carbon filter to remove any undesirable tastes, but these ones typically only last for ~100 L and new filters cost ~$30. They're heavy too and require proprietary bottles. Others do not contain a carbon element, but can filter 1000 L or more before requiring replacement. Two of the most popular options here include the LifeStraw and the Sawyer Mini, but the Sawyer product lasts longer (over 100,000 L) and could screw onto a standard disposable pop bottle, negating the need for heavy nalgene bottles, so I ordered a few from amazon.ca for about $25 each.
The Sawyer Mini and a 1 L disposable water bottle we use now. I've now used the Sawyer Mini on several trips this past summer and it is absolutely wonderful! It's possibly one of the best purchases I've ever made. The filter + bottle weigh just 82 g, or 13% of what the MiniWorks assembly weighs. Better still, it's ridiculously easy to use. I fill the water bottle in a stream, screw the filter on, then squeeze the bottle to squirt out water. I don't even need to remove my pack! On trails with lots of streams I can fill up at each one so I always have cold water too. While the filter hasn't yet clogged, it can be cleaned by backwashing with an included lightweight syringe. The only drawback to this setup is that after each drink the filter must be loosened to allow air back into the bottle, but the 5 seconds this takes is negligible. I very strongly recommend this setup to anyone tired of carrying the weight of a heavier filter and the frustration of pumping water, or really anyone that wants a simpler way to purify water.