It's something of a pity that a season that has really started to get a good sense of what it was doing so well ended up doing an episode that was pretty much exactly what Enterprise detractors had been saying the series was doing all along, a fairly unnecessarily indulgent look at established franchise lore.
There are certainly justifications, though they may be too subtle to appreciate for what they are. The detractors loved to say that Enterprise constantly risked contradicting canon by visiting certain elements that were historically known to not be known at this point in the timeline. The series had already danced around Ferengi and Romulans at this point, but "Regeneration" features the Borg, who famously catch Picard by complete surprise a long way off in the future.
Fans who don't worry about such things know that there are several clever things done in the episode. One is that it's basically a sequel to First Contact. The other is that it also provides the source of the Borg Collective's obsession with humanity.
Other than that, it seems like an excuse to feature the Borg again. Fans grew tired of them after Voyager used the Collective in countless stories. This is not to say that the Borg are not inherently still fascinating, but that many fans began to lose sight of their appeal, weakening "Regeneration"'s at least initial impact. The episode is also fairly generic in its storytelling, even if we get some updated perspective on their tech.
It's not bad but it's also not amazing, and sometimes the Borg just need to be amazing. That's the only real problem here.
franchise * series * essential * character
Bonita Friedericy (wife of "Phlox" John Billingsley)
Memory Alpha summary.