UPDATE! Sorry guys, I was out digging in the yard a lot longer than I planned!
After getting a couple quotes for 5-7K, I decided to go it alone. I think I ended up spending around 3K, but it took me 4 weeks of all my free time, and caused a ton of stress. In hindsight, I should have paid to have it done. But hey, I learned something and feel somewhat accomplished. The contractors agreed that running along my south property line (gravel drive) was the best way to drain water to the street. Since my neighbor is cool as a cucumber, he didn't care and signed a "neighborly agreement" to allow me to wreak havoc on his driveway for a while.
I ended up coincidentally meeting a ditch witch employee and he offered me a free weekend, which really pushed me in the direction of DIY. I gave him the details and he said "I'll bring you the biggest one we've got." Unfortunately he only had the second-biggest one available, which was 6 inches shorter. No big deal, right? Wrong. This ended up being 6 inches shorter than what I needed the deepest part of my trench. That meant hand-digging. Long story short: using the smaller trencher added several days to this project.
I was a little worried about the capabilities and operation of the trencher, but I must say I was pretty impressed. It was easy to learn and use, and pretty powerful.
Trencher / Ditch-Witch tips:
1. Unless your soil is soft and your tench is shallow, get one with tracks, not tires.
2. Go slow. Going too fast just means a more shallow trench and more loose dirt in the bottom. Going slow also helps you gently cut through roots without bouncing the machine all over the place.
3. Operation becomes significantly more difficult in rainy/wet conditions. (My yard wasn't waterlogged, but it started raining about 2/3 through the trenching and it got way more challenging. I should have taken a pic of it stuck. Had to pull it out of its own ditch with my 4runner.)
4. Operating off-camber (trenching accross a hillside) means that your trench is slanted. A slanted trench is much more likely to cave in. Mine did. A lot. It was somewhat of a blessing in disguise because the off-camber part was where I needed to dig my trench deeper, and it's nearly impossible to get in there unless it caves in. I will say that cave-ins probably added 24 hours of manual labor to this project. If you're going deeper than 30 inches, it's worth considering putting boards under the downhill side of the trencher (or getting a skid loader and leveling out the trencher path) to make the trencher cut straight down instead of at an angle into the hill.
5. Roots don't matter. If you get a somewhat larger trencher like mine, it can handle decent-sized roots with ease. I'd say I was cutting up to 4" roots and usually barely noticed them. I was considering removing the landscaping trees along my driveway and excavating the roots for fear that the trencher couldn't handle roots. No need. I ran the trencher as close to the trees as I could and had no issue. The trees seem to be surviving well, too, although I wouldn't recommend chopping a bunch of roots around trees that you truly value. (I like mine but if they died I wouldn't be heartbroken; they're somewhat overgrown and unruly anyway.)
6. Rocks do matter. Any rock bigger than your fist can jam the machine. No biggie, just reverse the chain for a second and the rock pops up to where you can grab it. But if you're in rocky terrain, you'll spend lots of time wrestling them out of your trench.
7. If your soil is clay like mine, and if you're making a deep trench like mine, you NEED a skid loader to backfill the ditch. (unless it's a really short ditch). Backfilling dirt and removing the excess was definitely the most labor-intensive part of this job. The clay here is some of the heaviest I've ever worked with, and each shovel-full is impressively heavy.
More to come, including pics!