By the time my dad bought his 1990 BMW 750iL we had already owned an '84 733i (which I now own), an '86 325, '87 325 and an '88 325. BMWs were nothing new to us by then and my dad wanted a new 7. In January of 1992 he came across a '90 750iL with 13K miles on the odometer, purchased new in November of 1990. Asking price was $43K. The original owner took quite a hit on his $70K+ luxury car. My dad bought it and still has it, refuses to sell it due to the fact that it a) has only 36K miles on it and b) it has been bulletproof reliable, even if it's mostly a garage queen.
From what we learned prior to buying this car was that the 750s to avoid are 1988 and 1989 models. First couple of years for any BMW is always kind of risky. First couple of years of initial 12 cylinder production apparently is quite risky. It is also important to note that any 750iL, regardless of model year, if not properly maintained will likely turn out to be a money pit.
The first thing that stands out in the 750iL is, naturally, the big 5.0 liter V12 engine. This engine is a gem. It sucks gas like a late 70s Trans Am but also has the ability to move this 4,235 lb vehicle like a muscle car. Many a time my dad has surprised a fast car owner with the power of the big 12. Even more amazing is its buttery smooth power delivery. You can quite easily be tormenting fast cars while Mozart pours out the speakers into its tomb-like silent interior.
In comparing the 750iL to a similar 735i (1988-1992), the first thing one notices is the smoothness and quiet of the 12 compared to the 6 cylinder model. Now, the 735i (or 735iL) is a wonderful car with probably one of the best engines BMW has ever produced, but it feels downright anemic compared to the powerful 296hp 12 cylinder. Aside from that, the two are very similar except for a few equipment and trim changes.
Reliability has been outstanding in the 750iL, requiring only new tires, a new battery and regular oil changes in the 8.5 years my dad has owned it. I have spoken with many a 750 owner with cars well over the 150K mile mark that still feel as tight and solid as the day they bought it.
This does not reflect the experience of other 750iL owners however. When buying a used 750iL make sure that all maintenance records are in order. If they are not, then you can be sure that the $8K flagship BMW you are looking at could very easily double that initial outlay of cash in the first couple of years of ownership.
That is the reason I would not recommend a used 750iL to a friend. If you are looking into buying your first used BMW, unless you have the history of the car since day one and an available warranty, stick with a 3 or 5 series car. Many a possible BMW fan has given up on the marque due to a bad 750 experience.