Despite the glowing advance reviews, "The Flash" is not the best superhero film of all time, the year, or even the month. But it is definitely a good movie overall. The humor is mostly spot on, the effects are impressive, and Ezra Miller is consistently convincing in the lead role. Michael Keaton still fits the Batman mantle pretty well, and Sasha Calle also delivers an excellent first impression as Supergirl. On top of that, of course, there are some neat cameos.
The pacing of the film isn't bad, either. It starts with a really entertaining sequence before the time travel plot picks up speed. The plot is comprehensible throughout and, above all, not as overly convoluted as in many other genre representatives. However, "The Flash" also has the usual problem of similar stories, that the fate of the characters never really gets to your heart, since it is not crucial for the "canon" anyway. But there is no such thing in the dying DCU anyway. In addition, there is not really a good villain, but then again, none was really needed here.
Overall, I would say that "The Flash" is neither a good conclusion to the old DCEU nor a good transition into the new DCU. You can still have a good time at the cinema, though, as the film works on its own and should appeal to fans of old DC works as well.
I just couldn't stop myself from comparing it to the animated Flashpoint Paradox. I really liked Flashpoint Paradox and for that reason I had somewhat high expectation from this story. This was fun at times but also somewhat disappointing for me.