A minor dissent here, not about the facts of Dan's post, which are correct, but about the conclusion that espresso should rest a few days. There's an equipment based work-around.
LM baskets extract evenly, with the colors lightening very slowly; whereas Faema and Cimbali baskets are much more "frontloaded," and have a rather sudden change from dark to light. This has nothing to do with double boilers versus HXs, but rather that the LM baskets seem to have more holes of a smaller size. The effect remains the same when the baskets are exchanged on the same machine. Moreover, since all three designs (I'm talking real LM ridged) hold roughly the same 18 to 20 grams of coffee, the baskets are very comparable.
If the color gets light fast on an LM basket extraction, the shot **will** be bad, period, regardless of the coffee's age. It's almost impossible to make a ristretto with one that isn't far harsher than a normale from the same coffee. The other two baskets work the other way, it's actually tough to make a good normale with a Cimbali or Faema basket.
This gets us to out of the roaster espresso. The taste variations when making ristrettos with a Faema basket are actually quite minor, the out of the roaster shots are slightly brighter and more aromatic, but no more so than if the coffees are brewed normally. Using LM baskets, yech!
Here's my guess: the conventional wisdom is that the later part of the shot is weak and bitterish, which is true for degassed beans. My guess is that the later part of an out of the roaster shot is coffee-seltzer, sourish rather than bitterish (although I've never tried this). This would explain why the change in taste balance is so dramatic pulling normales on LM baskets; while the dark stopped shots from a Faema basket are hardly affected.