Well, each edition of the game went through some changes, and 5th edition has made a major tweak in calculating Virtues & Flaws for characters...
...
BUT...
Most of the material from the earlier books (especially in terms of flavour) is still very much usable. Equally the monster/creature stats, while altering a bit here and there, are still very much usable. And the lab magic rules, while tweaked, are still very much of a kind and thus quite usable!
For example, there are two faerie supplements that came out for the game, one under 3rd edition, the other under fourth. Though they are on the same topic, the material is quite different in each one, partially due to mechanical reasons, but more importantly due to what each author wanted to emphasize. The amazing thing is that BOTH supplements are still good and compliment, rather than contradict, each other.
David Chart put out one supplement (
Heirs to Merlin) that has no "crunch" at all -- it is a history of England (to 1220), including many myths and legends, and the place of the Order of Hermes (the magi) in that history. Obviously this supplement is still useful for any edition of the game (as well as being generally useful for any game set in England during this time, much like the usefulness of many GURPS supplement).
If you pick up one of the adventures you will probably have to do some mechanical tinkering to bring it into shape, but I know they are reprinting one of the old adventures (
The Broken Covenant of Calebais -- amusingly it is essentially a dungeon crawl a la Ars Magica) in 5th ed format very soon (next month or two).
OTOH I would not recommend an otherwise quite excellent supplement,
The Wizard's Grimoire, as it is pretty mechanics-heavy and will probably be superceded by the new edition mechanics.
Just a few notions
