I put one in our 81 300D winter before last after debating on whether to install a block heater. (I have a factory block heater on my 90 Suburban, and it works great, as did my daughters on her 82 240D). But after reading all the posts on the horror stories of really tough installs and the possibility of cracking the block and it being hard to get to anyway, I chose to get a hose heater. (And I even had the block heater already!) I bought a Temro Zerostart, and it's a piece of cake to put in, and it does the job. No, it's not a block heater, but it works well enough for Missouri winters, which get down in the 0*-10* range for weeks, sometimes.
One thing I will say is that I had problems with mine starting last winter, and I was really disappointed, until I took the plug off of the aluminum housing and realized that when I had plugged it in, one of the male "spade" connectors had veered and missed the female part and ended up between it and the rubber mounting piece, so there was no contact. The whole last part of winter 08-09 it wasn't starting on the mornings that were below 17*, so I thought the hose heater was worthless. But after I refastened it correctly and realized that it HADN'T EVEN BEEN COMING ON BEFORE THAT!
, I found that it actually works really well. Never had a time after that point that it wouldn't start. Also, I bought a timer that I set to come on about 2 in the morning if it was going to get too bad so I didn't have it pulling 500 watts continuosly. (My truck's is 1200, I think) The hose heater doesn't have a thermostat, so if it's not down around 0* or a high wind chill, I don't want it sucking AC$$ all night long. Another thing you might want to check is to make sure you have an insulation pad inside your hood. I don't, but I'm getting one. When you come out in the morning and can see a melted spot where the heat is being lost, you know it's not efficient. And I'm a greedy guy when it comes to wasting electricity bucks. And, like lattitude500 said, get a new hose to put it in, unless yours is fairly new. It's an easy, cheap job. Why mess it up with an old hose. Besides, you'll always have an spare for a back up. There's a decent DIY somewhere on here that I used with good advice on exactly where to place it. (Halfway up the passenger side of the radiator). Yes, block heaters are probably better if you want to go to the extra trouble. But my hose heater works just fine, and only took me about 20 minutes to install, not including the flush and fill I did at the same time. And you will have to drain it, so you might as well flush it while you're there. Good luck.