It's funny that people love this film, and yet it is pretty miserable. Withnail is a pretty sad case, really.
There's something quite appealing about watching someone who is so deluded being put through the ringer. I love the sitcom "Black Books", and my favourite episode is where Bernard Black is locked out at night and has to wander the streets, begging food and having to demean himself in order to get shelter and warmth. The fact is he's such a self-serving oaf, you enjoy watching him being taken down a peg or two.
Likewise, Withnail could escape his desperate situation if he was prepared to start at the bottom rung of the acting ladder - taking understudy parts, and so on. But he's so deluded by images of his own greatness, the very fact that he's not prepared to lower himself means that when he's forced to by circumstances, it feels all the more satisfying.
Also, Withnail is an idiot. Although I love to watch him, I wouldn't want to know him. He's selfish, deluded, cowardly and willing to betray his best friend at the drop of a hat. It's like watching Big Brother. You keep watching to see how low he's willing to sink. What makes him a classic character is that he's also erudite, and able to sum up his thoughts in such a pithy and vivid way.
Plus, of course, it helps if you've been in a similar situation yourself. As someone else mentioned, many of W&I's fans are or have been students. Such people are generally intelligent and have been forced to live in relative poverty, probably with other people, and may have felt that the situation was beneath them. Even if they knew it was only temporary, they can see Withnail as a more extreme version of themselves.
A few years back, Empire magazine published a collection of essays called "Movie Heaven". There's one essay by Mark Ellen entitled "Drifting Into The Arena Of The Unwell" in which he draws parallels between W&I and his own student days. It's very funny. I recommend you seek it out. But there are parallels with my own student days as well. I was never an actor, but I did have a housemate who was gay and "in television". I never rubbed myself down with Deep Heat, but I did have to put my clothes on before I got out of bed because it was so cold. I never really had a problem with piles of washing up, mainly because I rarely ate anything sophisticated enough to require crockery. I was so removed from normality, I once walked 3 miles into the city centre and 3 miles back at about 2am in order to charge up the house electricity card so we could turn the lights and fridge on - and thought nothing of it.
So, yeah. To conclude with it would have been outstanding if I could have thought of a good way of succinctly encapsulating all my thoughts about the film in a single sentence. But I can't. So there you go. Life isn't like the movies, with their well written scripts, and expertly timed quips. Which is maybe why people like W&I - It's like real life, only funnier.