Sheldon Brown says any 8 of the 9 cogs of a Shimano 9 speed cassette can be used on any Shimano 7 speed freehub by using the spacers from the 9 speed hub. I am interested in this idea to widen the gearing (lower low) on my bike.

I was wondering of other's experience with this arrangement.

You just remove 1 cog and one spacer from a nine speed cassette. You'll need a nine speed shifter and a nine speed chain. Why not just buy a seven speed cassette with bigger cogs?

If I remember correctly it wasn't gearing but shifters that prompted the 8 of 9 on 7. 7 sp is still popular but shifters are hard to find. You can use the very popular and easy to find 9 sp brifters on a bike that was originally set up as a 7 sp without new rear hub or cassette hub body transplant.

It works fine. I've got about 10000km on a similar set up (7 of 9 + bb lockring on a 7s uniglide) and there are no issues. If you have the uniglide spline pattern it doesn't take too much effort to modify the splines on hyperglide cogs to work.

As for why, I did it because I couldn't find super-tight gearing (15-21) in 7 speed anymore.

The spacing of shimano 8 speed and 7 speed is not the same. An 8 speed cassette with one cog removed does not fit a 7 speed freehub. A nine speed cassette with one cog removed does fit a 7 speed cassette. That's the point of this discission and you've missed it.

Could I use my 600 EX 7 speed shifter with 8 of 9 on 7? If I wanted a larger than 28 big cog what would be the best derailleur to replace the short cage 600EX RD? My big cog now is a 24.

As I said, you have to use a nine speed shifter and a 9 speed chain. It will cost a lot of money. A 7 speed 14-28 cassette will not cost a lot of money. The only benefit of 8 of 9 on 7 is that you gain one cog. It's not worth it unless you already have the shifter.

What if I used friction shifters?

I picked up a 9 speed chain off Ebay for $10 and just use friction shifters. It's working out pretty good.

Can this be done with Shimano compatible 9 speed cassettes like SRAM