For me, it goes something like this. My 5 reasons for (and against) working agency:
1.) If you want flexibility in work schedules, agency is a plus. I like to go on a lot of vacations throughout the year. Sometimes, I may be gone for up to 20 days. A staff job really wouldn't work around MY schedule.
2.) If you are able to budget and save, agency may be good for you. I make 40-60% more than those I work side by side with (not including occassional overtime that makes it even more percentage wise). However, there is always a risk that I might not get the hours I want in a given week (sometimes even when I came off of a prior week with all the overtime I could take). I am prepared though. I have a budget. I live as if I made "staff pay". This allows me to save a lot of money. I keep a certain amount of liquid cash in my money market account that earns about 4.5-5%. Much better than a savings account though not quite as good a return as other investments, but since I have access to it whenever I need it, it's a good trade off. This is my rainy day cash. I invest the rest of my money in various things... a work-based 401k, stocks, CD's and so forth. Because I have followed this model for about a decade now, I could retire any time I wanted to. I work now because I enjoy it and because I continue to build a fantastic retirement for myself. I couldn't get the same results from a staff job.
3.) If you are flexible in "where and when", agency may be good for you. When I am not on vacation, I make myself available always. Of course, there are times when I have some other things planned, and I can't work. However, if for some reason things get a little slow, I make it clear to my staffers that I am available, and if they call based on my availability I TAKE THE SHIFT! A lot of agency nurses fail to understand the importance of honoring your availability that you report to the staffers. If you cry wolf too many times, they'll quit calling, and for good reason... They can't depend on you.
4.) If you approach agency work like you are self-employed, agency can work for you. Ultimately, you are responsible for your success in the agency world. Always leave a good impression with the facilities you go to. Make it known that you are excited to return and that you're available for more upcoming shifts. Don't be pushy just enthusiastic. You want to be REQUESTED when they have openings. I have worked with many nurses that I would consider more skilled than me, but due to my enthusiasm and willingness to go over and beyond, I am requested over them. You have to market yourself, and this is how you do it. Also, work on developing solid relationships with your staffers. Be honest with them. It's truly a "you pat my back I'll pat yours" kind of relationship. Make sure you're patting.
5.) If you like variety and can handle jumping into an often very heavy workload when staff may be quite short, overworked and grumpy, agency may be for you. Not everyone likes this situation. You may be at hospital A today, hospital B tomorrow and hospital C the next day. Can you handle working that way? Some like it, some don't. For me, it's great, but it is defenitely not everyone's cup of tea.
Hope that helps.