The Illusionist can be quite an interesting class to play. The general problem people have with it is that in a straight forward combat sort of way it is lacking. That is not the point of the class though. An Illusionist is a class that with a good imagination can do amazing things because they get all the neat spells for fooling your enemy's and they get some good ones before other types of Magic Users. I will go over a few spells the class can learn and some uses for them. I will be using Advanced Labyrinth Lord.
First up is Auditory Illusion which for an Illusionist a 1st level spell. It is an example of one of the spells Illusionists get before Magic Users as its a 2nd level spell for them. What the spell does mechanically is produce sound equal to 4 human sized beings +4 for every level above minimum required level that is centered somewhere within 60' plus 10' a level and lasts for 2 rounds a level. This is quite the useful spell for sneaking around and distracting guards or monsters.
Most people will realize the base uses for this spell but there are so many things to do with it besides that. If you get caught away from the party and are confronted by a monster you can claim that your friends are coming and make it sound like they are. You could scare the enemy's horses by having it make the sound of a wolf or some other predator common to the area. With a good imagination this spell can be of great use for many a distraction.
I saw Wall of Vapor and was happy. It is the only first level wall spell and I have an unhealthy fascination with wall spells. Mechanically it creates within 30' of the caster a opaque fog-like vapor covering 20' cube per a caster level that limits vision of all beings caught in it to 2'. Like most fog spells a strong wind can dissipate it before the end of the duration but as underground you generally don't face strong winds it will probably stick around barring enemy spell casters.
I find it so cool that it is a first level wall spell. It might not be as powerful as the higher level wall spells but it is quite useful to drop behind the party as they escape to stop enemy ranged units. A magic missile always hits but only if the spell caster can see you. When running away you can use it before a fork in the passages to obscure which way you run down. If you are somewhere you know using it to hide things like sudden drops and uneven terrain so that it trips up your pursuers is also an option. Really though stopping the ranged back up that your enemy's bring will be one of the better uses because it will force them to come into melee range of your fighter.
Finally the 3rd level spell Illusory Script is of some interesting use. With a permanent duration and at a range of 20' + 20' per a level the caster gets to write whatever he wants on any suitable writing material that is readable to only people who are designated at the casting of the spell. The script appears to be some form of foreign or magical writing and other Illusionist's will recognize it as illusory scrip and any unauthorized person trying to read it must save versus spell or suffer effects identical to the spell confusion for 3d6 rounds.
Because of the duration there are a number of quirky uses. An interesting use would be to use it on pieces of paper that you glue around the eye holes of your party's helmets so if stolen whoever try's to use it against you may end up incapacitated. If in a town and you have a guard questioning you for whatever, show them you "authorization" for what you are doing. Either it appears to be foreign script and nothing happens or they get confused so you can get away. On your magic scrolls you can put some illusory script labeling what's on it. You can read it but if someone else try's to use your scrolls they are in trouble. Trapping some of the pages of your spell book would also be a good idea. Basically anything you could write on becomes trappable.
When your group acquires permanent lodging such as a house when you are resting up there any day you don't do anything just throw some of these around to protect from thieves. By putting them on the outside of the window on where the window locks any thief trying to get in there has a chance of being confused and because they are not in a place you look at you don't have to go around dispelling them and recasting every time a new character joins the party. Putting it on and around keyholes would also have these benefits though you would have to be more careful as its around where anyone could see. If the house is your personal home though go wild. Remember to trap not only the lock but the hinges as well so they don't just bypass it to easy. This is a particularly useful spell is that unlike the Magic User equivalent of explosive runes the spell is not destructive so you can trap fragile items you don't want to have destroyed.
Finishing up I should mention that the Illusionist also gets Phantasmal Force in normal and greater version along with a number of cool spells like invisibility. Overall I would play this class if you want some interesting roleplaying experiences. While capable of combat Illusionists excel in trickery and misdirection. Imagination can make this a very powerful class.