Consistent success, finally!
Routine steps are quite usual - I pre-heat ROK and portafilter, grind beans quite fine but not too fine, and fill the portafilter almost full, then tamp well.

The most important points that made the difference vs a number of previous unsuccessful attempts are:

1) I have replaced the factory installed o-ring with a spare one, so that air does not escape from the top anymore.
Without having this done, nothing listed below would help as I think my ROK just could not develop necessary pressure. But still, this alone did not make my espresso good until I practiced further:
2) I fill up my ROK to the top of the water vessel.
The user guide suggests to fill to about 1 cm above the metal body edge. For me, this is too much air and thus too low pressure. If I do like it says, my espresso is bitter and has poor crema that is formed of big bubbles.
3) I use freshly roasted and freshly ground beans.
This provides great taste and rich crema, although after I figured out steps 1 and 2, I specially tried making espresso from older beans I used previously during my unsuccessful attempts, and I am now able to get quite decent shots with enough crema. Obviously, freshly roasted beans provide completely different experience and uncomparable taste, but these alone would not lead to success without steps 1 and 2 performed.

Thank you all for your valuable advices!

Thanks for the feedback.
I'm also making progress, but my shots are still a bit on the bitter side...

How much coffee do you use?

I've bought a naked portafilter and I see problems near the edges. I'm guessing the tamper is not tight enough and am waiting for a new one.

Just a word of caution: if you fill entirely the ROK with water, do not make the shot with your head above the ROK! I was doing the same thing and, on one occasion, the o-ring just went out, and the boiling water rushed up. Fortunately, it went directly to the ceiling, as I was in front of the Rok, not above it!

I just received this machine as a gift and have been getting some very grassy tastes with my darker roasts.

Does everyone generally do one pump or two? I have a proper 49mm tamper on the way that may very well fix all this.

I do a preheat with the portafilter in for twenty seconds or so. Remove and dry, fill, try to use the goofy scoop tamper. I have a 51mm tamper but havent overloaded basket enough for that to be effective. I've tried one and two pumps and didn't notice much difference.

I'm using a lido 2 on 9-10 notches. Anyone else use the lido 2 with this machine?

If your puck is below the ridge then that's less coffee than I find optimal for the ROK.
I generally find that grinding fine enough to need some pre-infusion pumps to be reliable for local beans (I have had a different experience from an interstate order) with up to ten minipumps before drops appear for fresh beans and 1-2 for older beans. For darker roasts I'll put in cooler water than the boiling I usually use.

I almost picked one of these up, actually an all-clad Pressi, at a thrift store, but it was in pretty dirty and questionable condition for the $50 asking price. I may need to go back in a week or two after the price drop to see if it's still around, but I really don't need any more espresso machines.

If you can bargain them down, I'd love to hear what you think about it. Most of the reviews I've seen haven't seemed to have had it dialed in.

The contraption is behind the counter of the boutique section of a Sally Anne, not known for bargaining. I'd have more luck if it were on the floor in the main section of the store, but then it wouldn't likely be complete. Worse yet, the people in the store know what it does, have had other people asking after it.

Well that's annoying!

aecletec wrote:If your puck is below the ridge then that's less coffee than I find optimal for the ROK.
I generally find that grinding fine enough to need some pre-infusion pumps to be reliable for local beans (I have had a different experience from an interstate order) with up to ten minipumps before drops appear for fresh beans and 1-2 for older beans. For darker roasts I'll put in cooler water than the boiling I usually use.

Thank you for this. So for fresh beans you do ten mini pumps? What do these consist of?

My minipump came from another forum member's suggestion - if I recall correctly, a half pump on a small lever to reduce channeling.

What I find works (ergonomics is a big factor for me) is a series of small light and short pumps from the fully raised position down to where there is resistance but no dripping of espresso.
These generally take less than a second, I think and what I find is that the beading appears more centrally rather than at the edges when performed well. I don't find that a single slow build up replicates this beading as consistently.
I find it difficult to give an explanation that I am satisfied with, but perhaps it is a more consistent method of preinfusion and/or a more gentle one.
While it does seem to cause more mess over the shower screen, I find that the results are worth the extra clean up.