With a good player and a good DM, 1e Illusionists could be absolutely killer. Like some of the other 1e classes (particularly the assassin) the effectiveness of the illusionist was often dependent on how the DM adjudicated the class's powers. The key issue was that, by the books, illusionary damage was real damage, psychic damage, if you will. Many, many, many DM's got this wrong, effectively hamstringing some of the illusionist's most potent spells. Also, there was the issue of how and when the DM rolled for saving throws.
The key to the illusionist was creativity. Most magic-user spells had a pretty set result after casting. The illusionist spells, on the other hand, were much more wide-open. In some ways, the illusionist could (appear to) do anything he wanted. Just at first level, Phantasmal Force could move walls, make the floor open up and swallow opponents, change the direction of tunnels, etc. As the illusionist got more powerful, his spells more and more shaped reality, and moved from shadow to substance. It was a very cool, distinct class, and not just a magic-user by another name.
R.A.