I just got done looking at Mars tonight. My best views ever.
I have a 10" dob with Zambuto mirror and an upgraded secondary.
I started at about 90x and kept bumping the power up. Surprisingly, the views continued to look pretty good during those moments of steady seeing.
Next thing I know I've got my 7mm Pentax and I'm at about 180x. Still looks pretty decent. Maria features are easily discerned and defined. In goes the 5mm Pentax at 250x. Still good.
I barlow the 7mm at 357x. This starts to look like the limit. But I try the barlow on the 5mm anyway, just to see. 500x was still usable, but overall the barlowed 7mm gave the best views.
I don't think I've been able to come close to using these types of powers before, so I thought I'd ask what the most usable power is that you've experienced and maybe how often you've been able to achieve that.