As others indicated, it was just a I-6 laid over on it’s side a bit. The reason why was to accomodate the intake/exhaust manifolds on the driver’s side of the engine. This gave the engine more guts than it would have had. It also lowered the cg and hood profile a bit, something that the V-6/8 types have been obsessed with for years.
As an engine, it was a gutsy little thing, with quite a torque-y snap to it. We had one in a Plymouth Barracuda. What I remember most about that car was that it was very snappy off the line and the huge, huge, huuuuuge expanse of glass in the rear window.
I’d go for a slant-six over any V-6 in a New York Minute.
Another advantage to the inline 6 is that they are very smooth. A properly-designed 6 cancels out vibrations up to the fifth harmonic order (look it up if you want to know what that means). A V6 basically acts like a pair of inline-3s; a single inline-3 rocks end-to-end and joining two of them in a vee cancels this by having them rock in opposited directions.
Also, even without tilting it 30 degrees off vertical, you can fit a less restrictive intake manifold. This, along with smoothness, is probably why BMW has kept inline-6s for so long.