This one is pretty average. Not great, not bad?
It serves to establish the time travel rules for new viewers - the past can be changed! Just because you know what 2005 looks like, you shouldn't assume it will look like that next time you visit.
The Doctor's edginess continues, turning on a dime to snap at Charles Dickens (and eventually turning back to apologize), and making it clear that he's perfectly willing to let the Gelth inhabit dead humans - it's like recycling!
I don't think the Gelth themselves hold together very well. They're stated to be gaseous, but they act more like stereotypical "non-corporeal" life forms for the most part. But the climax hinges on them being "drawn out" by the gas from the gaslights, so I guess they're attuned to that specific mixture, rather than a standard nitrogen/oxygen mix? Not really the sort of thing we should dwell on.
Of course, this episode also establishes the Cardiff rifts, and introduces us to Eve Myles, which will be important soon-ish.
I was originally going to comment on how much I liked it...but then the humanoid Gelth showed up, and looked pretty awful.
But I did like the earlier, wispy stuff, which reminded me of old-school hand-drawn animation.