I can't speak for others, and YMMV and all that, but I found the MM1 monsters truly unusable after mid-heroic. Before the damage errata, I had to throw much higher level monsters at my PCs to even have a chance of hurting them, with the well-known result of having two groups of combatants that could barely hit one another slogging it out with at-wills. The fights were farcical and boring. Even with the errata, you still have solos that can't hold up their end of the bargain. The original skill challenge rules were not quite as bad. I suppose they were playable if I didn't look at them too hard.
Really, with the original skill challenge rules, all the errata actually needed was a single footnote. They took that out
and adjusted them downward, which led to the challenges being too trivial; they then later fixed that as well as boosted the upper end of the DCs to adjust for the higher skill checks possible from additional options added via supplements. But at launch, that one footnote was the only real problem.
For the monsters... while I find the new ones an improvement (and maybe even
too vicious), the originals worked well enough, even in my epic game. I had a party that was pretty optimized, so we were already used to needing encounters a few levels higher to challenge them. We never ran into PCs and NPCs missing each other - by Epic levels, especially with Expertise in the system, the strikers almost never missed at all, except against the sturdiest opponents (higher level elite soldiers, for example). Maybe it was different at mid-heroic through mid-paragon (I was a player during that portion of the campaign), but it never seemed as noticably a problem as it has since been made out to be.
There were some disappointments, often with solos. I remember breaking out the Tarrasque and... never really having anyone too scared during the fight (aside from the initial emergence, and dropping one or two buildings on people during the fight.)
On the other hand, it wasn't a completely pushover, as it had been when I busted it out at the end of my last 3rd Edition campaign, and it got taken out in the first action of the combat by a shapechanged caster with multiple full-round charging pouncing attacks.
Still, as you say - experiences vary, and that is certainly true. But, at least in my case, I haven't found the base 4E rules as terrible as it might seem simply from looking at the errata. Some areas are serious fixes, but others are more minor tweaks. Which of course is its own question - would the 'middle ground' some prefer involve only making the serious fixes and not trying to perfectly fine tune the system? Maybe. For myself, I like them both, but I think it does make it seem like the initial system was more flawed than it actually was.
Maybe I'll limit my complaint to this: I don't think any previous MM1s needed fixing the way 4e's did. You had to realize that dragons were tougher than their CR, but that didn't require any work on your part.
Maybe... but I always felt the CR system itself was pretty dramatically flawed from the beginning, and never really got fixed. Particularly in the domain of the various rules for modifying monsters - whether advancing hit dice, adding class levels, or adding templates. Some really silly and absurd results would show up that, 'by the book', were supposed to be a certain CR.
Once I realized the best approach was to dismiss the entire system and just play it by feel, my games became a lot more balanced. With 4E, I really can trust the numbers. Whether the initial ones or the current boosted damage, it is much easier - for me as a DM - to feel confident in an encounter being appropriate.
Previously... well, I get that fights being hit or miss was an feature of the system to some folks, rather than a bug. But it was very frustrating as a DM to have such unpredictable encounters in the first place, and even more flawed guidelines on adjusted them to an appropriate level. And that was something that
never was really addressed at all.