There are Illusion spells out there I believe in the Spell Compendium that specifically makes illusions for flanking. With the shadow conjuration spells, the PrC's classes that deal with illusions and shadow magic, you can make a really powerfull class. The biggest limitation I can see comes from a freind of mine that played a 2nd edition illusionist alot. After a while, the dm brings in creatures that arn't fool by illusions. Or brings us up against casters with True Seeing. Eventually, he meshed conjuration well with his illusions, sometimes bringing in actuall walls and creatures. It ended up becoming a contest between the Dm and Pc in a battle of whats what.

Still, the general rule-of-thumb is...the bigger your imagination, the better your illusions.