My SteriPen Classic has been, overall, a pretty handy little device. Remember that it is only for certain circumstances -- it's not good for treating water that's not already fairly clear or may contain other contaminants like pollution, pesticides/fertilizers used in agriculture, and so on. So think about the water at your destination before you take the plunge.
That said, if you're looking for an easy way to treat high mountain streams or lakes -- water that's not likely contaminated with other particulates -- then the SteriPen is very, very handy. Mine has been a trooper, reliable and with plenty of battery life. I use those Lithium disposable batteries (my rechargeable ones haven't worked well at all with this unit), and one set (4) has treated several dozen liters of water with no trouble. Carry an extra set (they're very light batteries) and you'll be set for at least several days of constant use.
This would also be an excellent addition to your luggage when traveling to places where the tap water cannot be trusted, though I have not used it for this purpose.
And if you think you'll need to treat anything other than mountain streams, lakes, or foreign tap water, opt for another system. Think of the SteriPen as a sort of modern approach to boiling water -- if you wouldn't want to drink the water after just boiling it, then you won't want to drink it after zapping it with the SteriPen.
This combo set with cap and pre-filter is especially awesome, as it solves both the particulate problem with that little filter and the contaminated threads problem. Screw it on the top of your bottle and it covers all the threads; remove the little pre-filter after filling and the SteriPen fits snugly in its place so the Pen is mounted to the bottle, so to speak, and you don't have to hold it while it does its thing. Makes for a complete and very handy overall system.
Oh, and mine does not take AAA batteries, but rather AA batteries. Not sure why it's listed here as taking AAA, unless they changed the design. Most other SteriPens take those CR123 batteries, which are smaller but can also be harder to find. So, I prefer the AA battery version just in case I need a battery refill from that little backwater gas station -- or, more importantly, when abroad. Good luck finding CR123s in most third-world countries...
I have had one reliability incident with my SteriPen. My hands were just barely wet when I went to replace the batteries (stupid rechargeables, failed me again!!!), so the Pen got a little wet in the battery compartment and, presumably as a result of this, kept giving me a dreaded red light for the rest of that trip. So I packed it away, planning to send it back when I got home. But once I was back to civilization it worked again, so I hung onto it and haven't had a single problem through four other backcountry adventures, usually 3-5 days apiece.
So you do need to be a little careful with your SteriPen, and always bring tablets as a backup. I would also recommend carrying a little packtowel or something else to dry off the unit in case it gets wet. And if it stops working, open it up and let it air out. But that caveat aside, it makes for a fantastic and very, very easy way to make fresh water after hiking through the backcountry. If I'm going high in the mountains where I know the water isn't subject to pesticides or other sources of nasty pollution, I leave my filter at home and just bring the SteriPen (and some tablets, of course). No pumping, very little waiting, and no hot drinking water. Overall, very awesome.