Often, the first sip of an espresso is not exactly tasty to me and have a slight astringency to them, whereas the next sip and the rest of the cup will be really nice and completely enjoyable. Could it be my taste buds need to adjust to the explosion of flavors of the espresso, or is this a sign of a flaw in the extraction process? I sometimes feel it could be temperature related too (as in the first sip might just be too hot for my taste).

Does this sound familliar to anyone?

This has been a bone of contention in barista competition, where one sip is about as much as the judges can take without blowing out their buds in a tasting round. There is now a rule that the espressos must be stirred before being judged on taste and mouthfeel.

Most of the top baristas, iirc, lobbied for swirling, which I've come to hugely prefer when just taking a single sip from a shot. Perhaps, the WBC wasn't entirely impressed with the judges manual skills; "Is that a polka dot on my shirt or your espresso?"

With many espressos, the crema has the exact flavour issues your describe.
Swirling is one way to address this.
You can also do a "dodge shot" (where you let the first second or so of extraction pour into the drip tray instead of the cup) which seems to reduce this effect (though not eliminate it) with some coffees.

I would say the crema on my shots always tastes worse than the underlying liqueur. I would say ashy is the most prevalent flaw.

I don't find the flavor objectionable once the espresso is stirred, swirled, or transferred from shot glasses to a 5oz cup. If I pull directly into the cappuccino cup, I swirl the espresso before pouring latte art. Otherwise the first and last sips of the drink will be unbalanced.

I wonder if some equipment factors exaggerate this phenomenon; in my case:
- vibe pump e61 with slow preinfusion
- HX temperature hump
- flat burr Mazzer Mini that gives up a lot of solubles according to Jim's observations

It seems that all of the above factors contribute to a "front-loaded" extraction.

Could it be that a La Marzocco where the first drips are immediate and the temperature profile is flat serve to mitigate the crema balance issue?

Furthermore, wouldn't a "titan" conical grind tend to release solubles more steadily than my grind, as evidenced by a lighter-colored start of the flow and later blonding? (In contrast to my Mazzer Mini shots, which start dark and slow, then accelerate in the middle of the extraction.)

Or Malachi, are you saying that the crema generally tastes bad even in good espressos, so I should stop analyzing and keep stirring?

An arguments in favor of singles: Let it cool a few seconds and enjoy the whole enchilada.