I just got one installed on my LS about a month ago and everything is all GOOD! I live in San Diego so it doesn't get too cold but since I work a night shift, I just turn it on before I clock out and by the time I get into my car it's warmed up inside and out. Read up on your manual cuz there's a lot of options you could do to it. If you have a warranty and you're getting it installed, make sure they're a Directed Electronics certified installer just so your warranty won't get voided.
Also you're going to need a spare key to give to them cuz they install a chip in there, only for Fords/Lincoln. Sucks cuz they charged me an extra $50 for the chip. All it is, is that the key is placed in that chip.
I suggest you get it off ebay or do a search, you'll find one for about $160. I got mine installed for $200 all together including that chip. MSRP on the 791XV is $599. So you save a lot. Peace!