In Illinois, we have two types of depositions, discovery and evidence, with different rules for each. Both types of depositions can be used to impeach, but only evidence depositions, generally speaking, can be used as substantive evidence.
A new case from the 5th District illustrates how discovery depositions can rarely be used for substantive evidence at trial. It's a point worth remembering. In Berry v. American Standard, Inc., the asbestos plaintiff died before either side had taken an evidence deposition (pdf). After the plaintiff died, the defendants filed a motion to prohibit the use of the plaintiff's discovery deposition at trial "on the ground that Supreme Court Rule 212(a)(5) bars that use where the deponent is a party to the action." The circuit court granted the motion, and also granted a motion for summary judgment on behalf of the defendants.
In barring the use of the deceased plaintiff's discovery deposition at trial, the appellate court noted that none of the exceptions in Rule 212(a) applied (impeachment, admission, as an exception to the hearsay rule, for purposes for which affidavits may be used, or as former testimony if not by a party). The court also noted the purpose of the rule: "knowing in advance that a deposition is for discovery only and hence of limited availability, counsel ordinarily do not urge technical objections, and the taking of the deposition proceeds informally and expeditiously." The court also held that the discovery deposition could not be considered a "dying declaration."
Anyone with an interest in the difference between discovery and evidence depositions in Illinois is urged to read the case.