I finished reading The Subterraneans today. I had a false start with it last year, and a few stalls this time around, but I finally got into it about half-way through.
Kerouac really pulls you into his love affair with Mardou, with all of the accompanying passions and anxieties. Easy to relate to if you've had a love affair. Kerouac is at his spontaneous best here, and if I hadn't forced my way through Visions of Cody I may have given up again.
The nice thing about The Subterraneans, compared to Visions of Cody, is that it is shorter and, in my view, the plot is easier to stay engaged with. If you want to experience Kerouac's truly spontaneous style (much moreso than in The Dharma Bums or On The Road), I recommend this book as a starting point.
As usual, you'll need a key if you want to know what real-life people the characters represent. I was proud of myself for figuring out on my own (from Kerouac's description) which character was John Clellon Holmes. Here's a key (if you don't already have one bookmarked): Jack Kerouac Character Key.