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I used to have the 5mm Pentax XO. It was a superb lunar and planetary eyepiece, and I used it for DSOs too (esp globulars). The only problem was the eye relief: it was very tight at 3.6mm. So I've replaced it with the 6mm ZAO II, which has goldilocks eye relief, ie just right (for me at least), at a whisker over 5mm. That doesn't sound like much difference, but it is enough to make it much more comfortable for me. They are expensive, but they should hold their resale value well.
I would also agree with previous comments about the balance of qualities that the XWs possess (comfort/FOV/on-axis sharpness/colour purity). The XL predecessor models are also worth considering. They have 5 degrees less FOV, but are a great buy used.
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