Some pros and cons of expired domains are as follows:
Pros:
Previous owner choose the domain for a reason. I most probably have been a good market rich with keywords.
If the Domain Name is expired it must have at least 1 year of history. This is good for the search Engines. I lucked out and picked up a 10 year old domain once. On the First PR Ranking round it went straight to PR3 from PR0.
Some Domains Hold their PR. You may have seen around on Expired Domains Searches that some Show a PR Rating of the Domain. This can actually hold its place. So a PR6 you buy as an expired domain with a bit of work can hold that PR. A PR 6 site for example is worth $1000 easy on DropDay.com
Cons:
Cost can be very high. Make sure you know the value exact value of expiring domain. You can lose out by paying too much fees for it.
Fake PR Status. Make sure you check if the status of the PR Rating is correct. Many people buy a PR4 domain for $200 bucks only to find out it is no longer a PR4 so only buy high PR Expired domains if you know what you are doing.