Although I own a 530, this isn't a biased opinion- a lot of people prefer it over the 540 because the inline 6 is almost perfectly balanced, and doesn't have the inherent vibration issues that the V design has. The M54 is also considered by many to be the best BMW engine ever made, and was on Ward's 10 Best Engines list from '01-'03. But then who can argue with the M62's 282hp
I tried to do as much research as I could before I bought into an E39, and they all pretty much share the same problems. The difference I think was the changes they made in '01; previous to that the 525's and 528's had the same problems plaguing their 3 series counterparts. The 540 however is just a beast- the thing is a bit louder than the other 5'ers. I noticed that when I was test driving them.
And I hate the back end of the E34's- looks like someone chopped it off. Other than that, I think they're great. But the best looking BMW in my opinion will always be the E24