As more and more publishers become part of multi-national corporations, publishing contracts are becoming vastly complicated. Your agent will be there even if your book's editor leaves the publishing company. Agents who work on a commission don't earn any income if they can't sell your proposal. They have a great incentive to get the best possible deal for your book. Like marriage, an author-agent relationship is voluntary.
Financial solvency is important.
Agents are not immune to the vagaries of the economy. Whether an agency works with large, medium or small publishers is important information. An agent who has relationships with all three categories of publishers has an increased chance of selling any given book. Small publishers often are better at marketing certain kinds of books than large New York houses. What and how the agency earns its livelihood is something you should know.