This was an excellent episode. I loved pretty much all of it, I have no real complaints, just little nitpicks that don't really matter. Patrick Stewart was amazing, which was to be expected. Spiner did a great job as dream sequence Data. Even being noticeably older/more "chunky", his acting was still right on, and anything "off" can be explained as being distorted because its a dream. The main story has a good hook, and learning the full fallout of the synth attack from the short treks was interesting (although I kind of doubt that Starfleet couldn't have fixed Mars' problems at this point). Also, like others have said, I liked seeing a bit more of 24th century earth.
The few nitpicks i have are mostly just aesthetic based. I don't like the removal of the Romulan forehead bumps from all of the romulans. If DSC klingons have to look like shitty HR Giger abominations, they can afford to put bumpy foreheads on at least some of the romulans (we know they don't all have them, but come on, even ST09 showed some "regular" TNG-VOY era Romulans during Spock's flashback). That said, I'm glad they established that the real klingons are canon to this show with the Picard/Worf picture, so hopefully that's what we'll see if any klingons appear.
Also, the DSC ships (shuttles, the ship in the short trek, etc) had better just be the show being cheap, and not them overwriting actual 24th Century ships with the DSC designs. To be clear, I actually like the DSC Starfleet ships, and they would have made much more sense in the 24th Century, but they can't just reuse them now. also, the DSC Enterprise is not the Enterprise that Kirk commanded, so it shouldn't show up in that hologram in the preview/trailer for later in the season.
But, like I said, those are nitpicks. Overall I really enjoyed the episode and I'm completely hooked on the show right from the beginning, which is a nice change of pace after DSC took a whole season to get a grip on what it should be doing.