I might have originally missed the point of this thread as re-reading it, it seems you're asking about other generations of M5s as well.
E28 M5s have gotten past the point where the first question anyone would ask about one for sale is a request for compression and leak down numbers. Now, people just seem to buy the cars with little regard or concern for that. The cost of the engine overhaul hasn't decreased or become any less of a job, but for a good number of the cars out there, it has stopped being something that could in-effect total an M5. E28 M5s have jumped considerably in value. Some say it has to do with the E30 M3, or it is another effect of the Porsche market explosion dragging up the prices of German cars on the whole. The M5 is more exclusive than the E30 M3 and with much lower production numbers and a arguably lower survival rate (the low value of the cars for so many years caused many lesser examples to be parted and scrapped while an E30 M3 could go on to be a race car and still exist albeit with a different drivetrain or heavily modified), values seem to have increased at an alarming rate and even while M3s seem to have leveled off, that hasn't appeared to affect the M5 - at least some of the top examples.
E34 M5s make more power, but are also heavier and less-connected. Many suggest they're more attractive and they definitely have a different overall feel. I think it's actually more difficult to find a nice E34 M5 than a nice E28 M5. A lot of that has to do with the quality of interior components - particularly during the heart of the M5 production has made it more difficult to keep those cars in top shape. There are definitely some out there, but owning an E34 myself (a 525i touring), if I so much as look at a piece of trim wrong, it will break. That has tempered my enthusiasm for the E34 M5. They're still great cars and the last hand-built M5.
E39 M5s are much more common, much more complicated and insanely powerful. I bought one about a month ago and I'm still amazed by it, but I haven't had to really fix anything yet, so I'm not sure how long I'll say that, but the way this 4000 lb car drives is astounding in almost every way. I think they're currently at the bottom of their depreciation or at least holding steady and with the E60 cars being less-appealing to many due to ownership costs and relatively low numbers of manual transmission cars, I think the E39 will maintain a following.
Of the three generations I mentioned, I'd buy an E28. I still think that the E28 is one of the best cars BMW has ever built. It's not the best at anything but it does everything well. There's a reason it has the following it has.
I know I'm in the minority with my opinion, but I don't think the bumpers are too off-putting on an E28 535is or M5. That's possibly because the profile shape of the M5 is what drew me to these cars to begin with when they were new. While they definitely aren't subtle, I think they actually work and give the car a certain aesthetic. Again, I'm used to them and it's a position that's hard to argue. Of course, I probably shouldn't say anything at all, since I haven't put my money where my mouth is.
I take my position a little more seriously when it comes to a car like the M5, though. They made about 10,000 535is examples and that number is hard to pin down because they didn't have a separate VIN range, so at this point it's all conjecture, but people who were privy to sales info with BMW NA have suggested that the number is in the ballpark. The number of M5s that made it here is a little more than a 10th of that and there are probably 1/3 or so still around and believe it or not, a good number of the lower-end cars are going back to Europe to be restored there. Demand is high and it is increasingly difficult to find a nice car. While the Euro conversion is theoretically reversible, I am one of those that would prefer to see the cars left as-is. I also think that as the values either hold steady or increase (I mean just look at what EAG has been doing recently with these cars), original cars with the original US bumpers will have a higher value than a similar Euro-converted car. Neither aesthetic really affects how the cars drive and once you've driven a well-sorted example, you'll see why these cars have the following they do and why so many prefer their dynamics to the E24 M6.
Again, if you have any questions about anything E28-specific, I am slightly familiar with them and would be glad to help.