I have done a bit more scanning and I am pretty happy with the results at the standard 1mm elevation, which is the standard. I usually scan at a resolution of 4800. With three 645 negatives that tends to take about 10 minutes. I know I can set the V500 at 6400 but I don't think it is actually scanning at that resolution.
I choose to scan a 15mm by 15mm square out of the center of the 645 negative for the test. I could probably have scanned a smaller piece, but that size worked out all right. At the start of the test I could hold the film holder in the corner against the outside edge of the glass, but the edges are beveled on this scanner. After about 1mm of additional elevation the holder was above the bevel and then it wanted to move all over the place. I have shaky hands to start with and since I couldn't steady the holder against the outside edges of the scanner border any longer it tended to skate around pretty good. Not a lot of traction with those little nylon feet on glass. I really don't think it had that big an effect on the results since you can see a steady decline in focus quality as the elevation increased.
The holder is the Better Scan MF Variable Height film holder with the anti-newton glass. I haven't messed around at all with a wet mount and I doubt I want to go there right now.
I will say this much, I do like the V500 scanner. For the price I think it would be pretty tough to beat it. I may move up to the V750, but if I do that it will be to move up to the 4x5 scanning capability. Right now if I have a 4x5 negative that I really like, I send it out to my lab and have it scanned, which is a bit pricey.
I am also happy with the Better Scanning products. I originally purchased the anti-newton glass inserts for the 35mm holder and they were worth every penny IMO. I frequently use inexpensive black and white film and have ended up with some really curly strips of film that I would not have been able to scan if I had not had those glass inserts. Likewise with the Medium Format film holder. As it turned out I guess I don't need the adjustability, but that glass insert has also been valuable in holding down curly film strips.
I have read several posts where people say they do not want to have to keep the extra glass surfaces clean. I admit, the glass surfaces do need to be cleaned regularly but dust and scanning is a continual issue for me anyway so having the extra surfaces to clean just doesn't seem to create that much extra bother. I live in Nevada and dust for us is a way of life. I clean the scanner glass and the Better Scanning glass whenever I start, and several times during the process. I blow off and brush off the negatives as well and I have to say, those negatives tend to be a whole lot tougher to keep clean then the glass is. I spend seconds cleaning the glass and it can take several minutes to get my negatives clean enough to scan. The additional cleaning time required by the glass inserts just doesn't seem to be that big a deal.