I'm posting in reply to my own thread in case anybody is looking for the same information I was.
I ended up buying in the area of Lower Ballyfermot, aka east of the Kylemore Road and south of Ballyfermot Road, aka the Town/Inchicore end of Ballyfermot. (Not in fact between the Kylemore Road and LeFanu Road as is the title of this thread - although I've walked through that part a lot and it's lovely too, especially as they've just done a great job refurbishing the playground).
I can't speak to other parts of Ballyfermot (and it's a big place) but I've been here in Lower Ballyfermot a year now and despite the negative comments that I had found in some threads on this and other boards, I have had only good experiences of the area.
There are parks, playgrounds, shops, as well as several gyms and a swimming pool (er, so my housemate tells me, I'm more of a couch potato myself). The main Ballyfermot shops are great too, a Tesco, a bakery, clothes shops, a great veggie shop and a butchers people come for miles to buy from. The area is very mixed in terms of single people, young families, teenagers and pensioners, and it's a mix I like a lot. The teenagers are not scary, they're just doing their teenage thing and pretty much minding their own business.
The bus routes are excellent (and although no I don't sit down the back upstairs on the 78/a, I do get it) the 79/a is great, you're in town in 20-25 minutes even in rush-hour and it's cheap to get a taxi home if you're out at the weekend. My neighbours are the absolute best - friendly, welcoming, interested but not intrusive and incredibly helpful when I needed it.
I've felt safe coming and going at all times of the day and night, and my flatmate (I rented out a room no problem) has had her bike locked out the front for the last six months without it ever being taken (and I didn't think that was possible in any area of Dublin these days).
As you might have guessed by now, I love this area and I love living here. If anybody is thinking of buying here, especially as there's such good value at the moment, I would recommend it without hesitation, I hope you wouldn't be put off by the kind of posts that made me nervous when I was considering buying here, as I say I think it's the gem of Dublin.