Do scientists using user facilities (such as Advanced Light Sources) have to travel frequently? I recently met an experimental physicist who does his measurements using the X-ray beam-line at a national lab. Since the university he works at is not too close to the facility, I wonder how much travel do people in this and related fields of research have to do for taking measurements on their samples. 

Also, do people have any personal opinions about how strenuous the constant travelling can be?
I did my PhD in experimental nuclear structure physics, in a similar situation to the one you've described here. Within a month of starting my PhD, I was sent to the IFIN-HH lab in Romania for a week or so, to help a collaborator with their experiment. A few months later, I traveled to the experiment which provided my PhD data at Argonne National Laboratory in the US. This level of travel kept up for the 3.5 years of my PhD, to Germany, Italy, Switzerland, France and Romania, all on multiple occasions, although it tail off a bit towards the end when I was writing up my thesis. I have to admit, the travelling did get a little tiring, but I generally enjoyed the experience.

So not only are you attending your own experiment(s), but also those of collaborators from both your own institute and others. I was ok with this as I wasn't in a long-term relationship (well, I eventually was for a while, but with a PhD student on the same course), and didn't have children, so had few commitments which required my regular presence.

Really what a person looking at this level of travel needs to decide is if they actually want to travel this much, and if their commitments can handle them not being at home for days or weeks at a time, on a regular basis. (Admittedly, this is much more thought than I put into it at the time!)