To generalise brutally, my impression is that a majority of the fanfic side of fandom was appalled, while the more literary sf side was generally fascinated, with plenty of exceptions on both sides. To summarise reaction from my f-list (a number of these posts are locked, so you'll have to take my word for it):
Like it: calapine here, pickwick here, clanwilliam here, redfiona99 here, major_clanger here, Fiona Moore, I think jinxed_wood here, more or less paratti here, parrot_knight here, ephiriel here, I think sunnytyler001 here, strange_complex here, surliminal here, lonemagpie here, nostalgia_lj here, and lilaeth here.
Don't like it: irishkate here, birdsflying here, mraaltariel here, nowrah here, irishkate (firstly) here, altariel here, ladymoonray here, and sammywol here.
Further brief comment without saying if they liked it or not: miss_s_b here and dougs here.
My own take: Count me on the fascinated side. Yes, it was derivative - particularly of the 1979 Quatermass, and the last scene being I admit more Douglas Adams (without the humour) than Acts 1:9 or E.T.. But, you know, that's genre for you; and Quatermass and the Hitch-Hiker's Guide are firmly established as major moments of British media sf, so it's entirely understandable to try to moor your show in that tradition. Anyway, much great art is derived from other sources; the question is, does it add anything, and did it work dramatically?
My subjective answers are yes and yes. What was different this time was the radical step of actually developing Jack's character. Tom Baker (most eloquently among many) has pointed out that the Doctor can never really change; there is always a reset button at the end of each story (and RTD has nibbled away at the first of those but not really the second). Jack, on the other hand, has now lost his lover, and then become a monster who sacrifices his own family, without their consent, to save the world; and now has to live with the knowledge of that action forever.
It worked dramatically for me largely because of the guest cast. Peter Capaldi as Frobisher in particular, but also Paul Copley as Clem, Nicholas Farrell as the PM (great choice of name, almost-but-not-quite Gordon Brown), Cush Jumbo as Lois, Susan Brown as Bridget, Lucy Cohu as Jack's daughter, Ian Gelder as the sinister Mr Dekker and Katy Wix and Rhodri Lewis as Ianto's sister and brother-in-law. the regulars all seemed to me at the top of their game as well (and I don't share the view that Barrowman and Lloyd can't act; they were certainly doing so on this week's show). The fact that the actors clearly bought into the world that Davies and his team created made it convincing.
Was it manipulative? Well, of course. But I felt that some of the worst characteristic excesses of New Who were dialled down a bit here so that the acting and the script could do the work - thinking particularly of Murray Gold's music, which occasionally has had to do the work of telling us how to feel when the rest of the show wasn't up to it; also Davies has got much better at the pacing of his own scripts. Tony Keen has been cruelly accurate in describing some of RTD's other work as resorting to Total Bollock Overdrive; I thought there was very little of that here.
It's a subsidiary issue, but I did like the political parts of the show as well. There was a fascinating contrast between the largely static scenes of Whitehall (be it Downing Street or Thames House) and the dynamic hustle of Cardiff. Of course in Real Life, the government would not choose Frobisher's children as the public examples, or hand out secret passwords to the new girl on her first day, or surrender quite so abjectly sovereignty to the Americans; but in Real Life they aren't in touch with green aliens who survive on poison gas either. At least we hope not.
I can't see there being any more Torchwood after this. The Hub is destroyed, Gwen presumably back with the police once she returns from maternity leave, and Jack off exploring the universe and dealing with his own demons. To reunite even Jack and Gwen (let alone Ianto) would require a reset button which is surely beyond even the powers of Russell T Davies and Steven Moffat combined. But we shouldn't mourn; it was good this week, and I may be in a minority but I enjoyed the first two series as well. Sometimes it's good to go out with a bang.
Edited to add: For more reactions see here.