The only "cons" I could discern from my time at Express Pros are perhaps these two (which aren't really "cons" at all):
1) You WILL be employed quickly. If you are a slacker or someone else is forcing you to "get a job," then you will have to change your attitude. Otherwise, you will be like some people I have met who show up to a job that Express found for them, make excuses, and then not show up, burning the bridge. So only use Express Pros if you are serious about finding work, because your 24-hour days of video gaming and television will be over!
2) I have estimated that a great majority of jobs through Express Pros are going to be dead-end jobs. By that I mean that your likelihood of growing with a company and advancing with them is rather hit-and-miss, especially in terms of your "dream job." Express Pros has a lot of warehouse, light industrial, and administrative positions available, but not all of them will take you places. So Express Pros is really more about finding you work "now" than "forever" at one place. Of course, there are exceptions to that generalization, such as what I eventually found for myself, though I am working in an industry that I am starting from scratch in. The possibilities are out there, but most of these jobs are ones with either high turn-over, kind of unwanted jobs (low skill, low experience, low education), and/or short-term. My recommendation is to only apply for positions that you will commit to and see where they go. (Of course, all of that is how it is at most temp agencies everywhere.)