I own and have used all 3 of the major 10-stop filters. (BigStopper, Hi-Tech, and Sing-Ray).
I can safely say, the Sing-ray is the best of the bunch. It is an excellent piece of glass and fits very well in the holder.
- The Flange around the outside covers the entire area, so there is no problem with slight mis-allignment causing a light leak (a problem with the LEE)
- I have noticed it can be slightly warm. (50-100 degK or so), but no discernible colour cast, certainly not the horrendous blue of some others.
I recently took the SinghRay 10 stop and 5 stop More-Slow through Iceland, and they performed very well with one exception.
The 5-stop is paper thin, and very fragile. It broke in my case near the end of the trip, when it was between two other filters. I still have no idea how.
The filter itself was excellent, and performed very very well as a filter, but I just can't recommend it due to fragility.
The 10-stop is normal thickness and works just fine.
The lee Bigstopper is pretty damn good, but I prefer the slightly warm of the Sing-Ray, vs the slightly cool/blue of the Lee.
The Hitech, I am afraid is, in my experience, complete rubbish. I have used/seen 3 of them, and they all had a horrendous colour cast, especially during long exposures. It is now a coaster.
Here is a blog of another comparison: http://blog.jholko.com/2012/06/24/battle-of-the-big-stoppers-lee-vs-hi-tech-10-stop-nd-filter-showdown/