Well, the first episode of the new series. A new Doctor, the first episode out of the solar system. So many potentially interesting new worlds to explore.
Russell T Davies, scripting, of course, disappoints.
There were a few good things about the episode. The opening scenes were nice, Mickey's lingering resentment at Rose vanishing off, and the apple grass, and even the New^n York.
The Doctor's initial investigations of matters in the hospital were well acted and scripted, for the most part, although there was a wee bit too much infodump given to the poor actress near the Face of Boe.
For the most part, though, the plot and design was lacking in rigour and depth, especially in the second half. The plot was obviously intended as a comment on animal testing, and the ethics of medical research. Unfortunately, this was hamfistedly done on multiple levels: it is not obvious why a bank of humans infested with all known diseases would massively accelerate the development of cures to specific diseases (one would think that the interactions between the pathogens would somewhat complicate matters); as Cassandra-in-Rose questions, and isn't properly answered, shouldn't the test humans be, well, dead from their massive infections?; why were the clones grown with brains in the first place? It seems that most diseases aren't dependant on the sentience of the body they infect, and this would avoid the conveniently present ethical issue which is so hammered into the episode; how the hell can these clones even speak? The Spokesclone gets a comment along the lines of "being part of the machine, we know what the machine knows", but that's just weak handwaving to cover up a major flaw in the exposition and design. There's no good reason why they should be conscious, rational (if obviously zombielike) beings, other than that it suits Russell's purpose.
The infectiousness of the plague-clones also has some odd anomalies: the cat nurses seem oddly susceptible to many of the contact-infectious pathogens, despite being an entirely different species (the difficulty that H5N1 bird flu has crossing the species gap shows how unlikely that is); the clones are also, strangely, only contact-infectious. We are, presumably, to assume that all air-bourne infectious agents were wiped out in a previous era?
And then there's the resolution, where the Doctor mixes a ton of cures for various infectious diseases together, and they somehow cure all the clones. Considering the complex interactions between treatments for modern diseases, one would think it just as likely that they'd kill them, or make them turn blue and speak esperanto. Basically, it's a cheap cop-out to a cheaply plotted episode.
Talking of cheap cop-outs, that's a fairly good description of most of the second plot-thread, with Cassandra hopping bodies merrily. From Cassandra's sudden change of heart with respect to her body-chauvanism (suddenly she's only too glad to get out of her brain-jar and skin-trampoline body into something with a skeleton), to her equally sudden change of heart at the end (from hard-bitten life-long survivor to world-weary welcomer of death in a single bodyswap and a bit of a woozy feeling), the characterisation is laughable. And let's not even mention the embarassingly bad acting of Tennant as Cassandra-in-Doctor (didn't the "I'm a woman in a man's body so I'm limp-wristed" thing get old sometime in the Stone Age?), or the various niggles with the bodyswap process (from the "soul" emanating from Cassandra's trampoline in the initial process, rather than her brainjar, to the issues over whether she should even have a chance of possessing The Doctor, who has an entirely different neural structure, presumably, to a human), or Billie's somewhat odd attempt at a posh accent.
Other than that, the only other niggles are the obvious reuse of the Nestene Consciousness' lair (from S27, E1) as part of the storage facility for clones, the surprisingly uselessness of the cat nurses considering their feline ancestry, and the oddly unsatisfying hyperactivity of Tennant's Doctor. (We're told that the Face of Boe will reveal his/her/its wisdom to a "wandering god". I wasn't aware that ADHD was a qualification for godhead.)
So, yes. 3/10, must try harder.