I've used a Steripen Adventurer since 2008, a model that I don't think is made any more. Basically it was smaller (therefore lighter, and also less money) than the standard one at that time.
Prior to that I used iodene tablets, which were a bit of a faff because after treating your water you have to wait ten minutes before you can drink it. The Steripen is much simpler, one press of a button for 1 litre, and a little green light starts flashing, then you have one minute to start treating your water. A UV light glows on the tip in the water (I've never timed it, but I count slowly up to about 150) and you can use the water as soon as the UV light has gone out.
I don't know what the current models are like but the standard one I mentioned above clipped onto a wide-necked 1 litre Nalgene bottle and you could shake the pair about. You can't do that with my Adventurer, so I just stir it about in the bottle, one way then the other way.
You can also buy a Steripen Prefilter that clips onto a Nalgene bottle, this makes sure you don't get bits of algae and sediment in your water when you fill the bottle from a stream or lake. The only problem I had in 2008 was that there were a number of UK sellers of the Steripen but no-one stocked the Prefilter so I had to order it from the USA, which incurred hefty postage and a long wait.
Much as I like my Steripen, there are some things to be borne in mind:
You must replace the batteries every year. Mine takes 2 x CR123 or CR123a button batteries, easily obtained on eBay. I failed to do that once, thinking it would be OK as I'd only used it on two trips the previous year, and it died on me. Some people don't like Steripens because you are dependant on batteries.
The tip is glass, it's probably quite thick but it wouldn't do to drop it onto rocks.
During treatment you must keep the instrument's electrodes immersed in the water otherwise it will stop working, consequently I have to grip it with just my fingertips. I am sure the time will come when I drop it into the bottle!
Last but not least, the Steripen only renders organisms harmless to make the water safe to drink. It does not remove chemicals, pollutants, etc. However I've suffered no ill effects whatsoever in ten years of use of mine.