They’re legit, but you won’t make much money and are perhaps not the ideal choice for a Master’s graduate.
The PIRGs are a great way for a college student to learn about political campaigns, have a lot of fun with the people there, develop personal skills and indeed do real good for the world. Not a good way to make money; count on making less than the lowest amount they’ll quote you, but if you can possibly afford it, it’s a great experience for a summer.
Based on the corporate culture of a decade ago, I don’t recommend them much as a long-term career choice (i.e. becoming a canvass director or anything).
Canvassing is not a career killer on your resume, but canvassing is a little below the level expected for a Masters graduate, basically half a step above volunteering somewhere – it shows interest on your part, and some minor experience, but since anyone with a pulse can get hired, it doesn’t prove you have any employable skills or talent. So you may decide tactically to leave it off of your resume just to be more impressive.
Likewise, if you want to work for non-profits, some kind of fundraising experience is always valuable, even if it’s not the kind that will get any particular respect from real fundraisers anywhere.
I’d say your attitude of ‘get a little exercise while I search for a real job’ is about right, as long as you don’t need to be making much money. The great thing about it is that it can be incredibly flexible with days off, so you can do interviews and other job search stuff.