Powerline can be effective
a) The signal will leak out, but attenuate. Add a password and no one should be able to piggyback. I've found the software pretty easy, although if your device has a number for the security protocol, note it down BEFORE you plug it in.
b) The 200mbps or 85mbps is the raw rate ie you'll get less due to interference and the overhead of the protocols, but that's the same for 54mbps etc for wireless. HOWEVER the 200mbps is shared by all the devices so if you have several boxes all transmitting SIMULTANEOUSLY then they are sharing the bandwidth. Again this is the same situation as wireless, except if you re in a block of flats, there are a whole load of other people sharing it. There are no channels to avoid interference, but 85mbps and 200mbps exist at different frequencies.
c) It may not work across multiple fuseboards. My sister has a large house with three separate fuseboards and the signal will not cross. Most houses have a single fuseboard and your upstairs downstairs ring mains will just be separate circuits (fuses, breakers, rcds) on the same fuseboard
d) You can fiddle with placement to improve signal quality, ie direct in the wall, on a distribution block... Software will be provided though as someone mentioned it may be windows only.
e) I have found that sometimes, 200mbps doesn't work where 85mbps does - don't know why.
f) Its great for bridging, ie plug it in to a router, or combine it with wireless access points. Sure there are bandwidth limits, but unless you are shunting multi gigabyte files, or multiple 1080p video streams it'll be ok.
g) It really is easy to set up and is a breath of fresh air after wireless.