I saw this thread over the weekend and wanted to respond as this is something I have a real thing about - but I was only online on my phone over the weekend so only getting back to this now!
I have always felt that backing into a space is more difficult, as one has a narrower area to fit into, whereas backing out, in theory, you have a much wider area to reverse into. That's not of course to say you should not observe what's around, or that you have carte blanche to take up all the available space, but the bottom line is I find it a lot easier and quicker to back OUT than I do to back IN. It's the way my head works, but I would never condone backing out without making full observations.
This was then discussed on Top Gear, and I was alarmed to discover they felt the opposite. While there's a lot in that show people might not agree with of course, they are all 'professional' drivers, so I attached some credibility to what they said on this issue, and their take on it WAS down to the issue of it being more likely that observation is harder if reversing out due to the range of possible directions of traffic and people approaching which could be out of your line of sight, whereas reversing in, really the only concern (barring freak accidents) is avoiding the other cars.
However, I do still see a lot of people taking a LONG time to reverse in, and it does irritate me, I admit. I drive in, and when I reverse out, I do so smoothly and quickly (and with suitable observations to be safe).
Oddly enough, while pulling through is not to my knowledge illegal here in Ireland (where I live) or the UK (where I learned to drive), it has always felt a little bit rude to me. I'm just not sure why!!