I have no idea what to think about this. The first episode I didn't think was very good at all, but the second I actually rather liked (despite plot misgivings) and the third was pretty good also. My biggest criticism is that, because of the way the plot was constructed and because I went in with (I think sensibly) low expectations, by the time I realised I was watching something that was actually quite good, it was practically over. Had this special been done maybe after series six then I'd have gone in with high expectations and been thinking 'how are they going to explain this?' rather than 'oh, God, what?' (although then I'd have been slightly disappointed). The tension is diminished when I don't really expect it to resolve sensibly.
Overall, I thought it was a basically pretty good show. Not really 'Red Dwarf' as I understand it, but it can't be that without Rob Grant. That said, there are a number of things in the script that grated with me, including continuity errors, nonsense, plot holes, and worst of all, one major break with character. On top of that, the whole thing just wasn't as funny as Red Dwarf should be, and some sequences were utterly dreadful. It would have really benefited from a couple more drafts, preferably including a fresh pair of eyes to rein in some of Naylor's worst excesses, and it would have been far better off presented as one long feature — not least because I'm told ratings plunged from more than 2m to less than 1m literally overnight after part one. For the sake of a couple of writers for a couple of months, this show was crippled and came across as unfinished. Artistically, that's tragic, and I'm not sure it was even wise commercially: the ratings drop after part one must have hurt Dave, and I'm sure they'd have saved money in excised SFX follies anyway.
Hopefully if this is the lead-in to a new series then these mistakes will be learned from. If not, then it's a better swansong than Only The Good.
Related link: this review I agree with almost entirely.