I have a 14' CTD, bought it new and I now have 63k miles. Easily one of the most underrated eco cars. Marketing was poor, probably due to the last GM passenger car diesel debacle. I believe this car was really more or less to test out the market and get some real world testing with a clean diesel that actually meets the EPA standards.
I mainly commute up and down the parkway, so my avg is 42 mpg. If you know the parkway between exits 160 and 130, it's usually heavy traffic both directions, especially in the oranges. With a light foot and refined techniques I can get 45mpg on a tank WITH traffic, including regens. In a more rural setting I know I get even better. I usually go 600+ miles between tanks not trying, so every like 8 days I fill up. Some people have squeezed 1000+ miles on a single tank which is better than 55mpg (epa rated 46mpg highway). Impressive considering the DPF/SCR epa nonsense. So it will EASILY surpass your 30mpg mark. And diesel is almost as cheap as regular. Considering most new eco cars have turbo charged setups requiring mid-grade to avoid knock/timing retard, it's worth the money. (Yes, you can run regular in these newer turbo cars, but you won't maximize efficiency)
Overall the car is well built. It only came as a top of the line model, so you'll only find leather seats with navigation, remote start and all that with a 6spd auto. My Link nav is probably my favorite of any brand I've used. One cool feature for road trips is it will display points of interest at the next three exits along your planned route on the interstates, really useful ! It also includes a lot of sound deadening due to the diesel motor so the cabin is significantly quieter than the gas variant or other 4door economy cars (speaking from experience). The ride is quiet and since it's a bit heavy you won't lose your teeth hitting a bump like some cars. The power is great, 264ft-lbs with 280ft-lbs overboost for off the line, pushes you into the seat! Kinda hurts my mileage lol Turbo is oil cooled variable geometry, so it spools up quick. One guy has over 200k and he only replaced two exhaust sensors, both under warranty. There are several with more than 100k, no issues. So reliability is up there. Although reviews may be poor due to the numerous exhaust sensor issues...
With that said, the most common issue(s) and what I've dealt with are exhaust sensor failures, but they're pretty much all covered with recalls (premature failures due to exhaust humidity from SCR/DEF). The DEF tank heaters failed in several cars, but that is now covered under a TSB, so no charge. Other than that nothing major, some oddball things surface on the forum here and there, but they're usually covered under a warranty. As I said, since this car seems more of a test mule than anything, any issue the dealer works on is usually resolved with GM TAC engineers. Most of the exhaust sensors have been updated. Last week I had a CEL for the particulate sensor and it was replaced for free (covered up to 10 years or 120k). They also reflashed the ecm with their newest software to help resolve issues some were having with NOx senors.
As far as the exhaust related issues, if you had issues that werent covered or dont like the EPA riding shotgun, there are two companies who make dpf/SCR deletes, about $1000 I believe. Since all these new sensors now have their own module attached, they are about $200 a pop (again, pretty much all under warranty) the tune would pay for itself in the long run. Only issue is only gm can reflash the factory tune from what I read. But if you choose to go that route you probably won't go back anyway.
In summary... it's a great car, excellent mileage and you could probably score one cheap due to the exhaust issues (which are mostly covered afaik). It's worth seriously considering.
Info on the LUZ engine (made in Germany, proven design in Europe that gm reworked to meet epa)
http://gmauthority.com/blog/gm/gm-engines/luz/
Cruze with delete
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https://youtu.be/4JrfGIWVC7U