I purchased an L-eXtreme filter a while back but returned it after hearing about halos and problems with fast scopes. I'm planning to purchase an f/5 scope and potentially use a reducer to bring it to f/4 so I didn't want to have to deal with those issues.
The IDAS NBX filter is a similar filter, but with wider bandpass at about 15nm compared to the L-eXtreme's 7nm. That added width, plus their design which places them off center a bit to allow for more bandpass shift at fast f/ratios, means they are touted as working with scopes as fast as f/2.
I compared the two filters on Astrobin and found halos on both of them in some situations, but it seemed like the NBX was perhaps a little better in that regard. So I ordered the NBX which, like everything these days, was on long-term back order.
I just received it, and discovered they are now shipping the NBZ, not the NBX. According to one website, IDAS redesigned the filter "from the ground up" to significantly reduce internal reflections and eliminate halos. Cool! Just wanted to post this in case others are looking at these filters. The L-eXtreme's narrower bandpass will of course provide a stronger effect and a darker background. But the 15nm NBZ should still be pretty good. And if it both eliminates halos and works with fast scopes, that seems like a good tradeoff to me.
-Dan
