William Optics are fast becoming one of the most popular astronomical companies as their series of telescopes and accessories share a high level of quality at a reasonable cost. The SPL series are a great example of WO's commitment to producing fine equipment, the 6mm itself excels within the SPL line.
William Optics have used rather a lot of lenses (or elements as they're known) in the SPL's construction. There are a total of 7 lenses arranged into 3 groups of 2 and a single one. Usually the addition of glass in an optical system can only deteriorate image quality, WO have used a high quality glass to minimise image degradation and the result is a high magnification eyepiece that retains a decent eye relief.
Eye relief is the maximum distance one can go from an eyepiece lens and still take in the whole field of view. Usually eyepieces of a high magnification have a very small amount of eye relief but due to the extra lenses used the SPL has a mighty 20mm which makes it comfortable to look through and is also suitable for spectacle wearers. When coupled with a large objective lens (the one you look through) the SPL is certainly a joy to use from a comfort point of view.
One minor complaint is the 55 degree field of view which for a planetary eyepiece with so many lenses is rather on the small side. However this is the only aspect of the eyepiece that could have been improved upon, optically the SPL is astounding for the price.
Each lens has been blackened at the edges to improve contrast and has been fully multi coated, a good lens coating improves light transmission meaning the object you are observing will be as bright as possible.
The whole SPL series is par focal which is a really welcome feature, this means once perfect focus has been achieved with one SPL you won't need to adjust the focuser at all when swapping to another SPL.
Something to consider though is the SPL is heavier than your average eyepiece due to the extra lenses used, this could cause some balancing issues when swapping between eyepieces, this is merely something to be aware of and is more than made up for by the pure brilliance of the SPL both visually and from a comfort perspective. A fantastic eyepiece that can be snapped up for £70.