If you're not familiar with Rilo Kiley, I sure as hell wouldn't start with this one. Although More Adventurous, the last album, has some country/folky moments, there is rock and pop on there as well. It's a good sampling of what they can do.
Under the Blacklight...I got this a week ago and I'm a little torn, not so much "sellout" as "what the fuck were they thinking?" There are like 5 great songs here and an equal number of "meh" ones. The Fleetwood Mac thing that the guitarist sings co-lead vocals on? Dreamworld? No. Just...no. That Gloria Estefan thing that has Spanish enunciated like a gringo? No. I'm not a big fan of The Moneymaker either, and if that was some kind of ironic pre-emptive stike against cries of "sellout!", it doesn't work that way either.
What's good is very good; Silver Lining, Breaking Up, the title track, Smoke Detector, 15. It's just that it's very lightweight. You can have minimalist lyrics and still have more resonance than this album. Musically, it's all over the place, going from Motown to 80's synth pop to 50's rock n roll to latin pop to god knows what.
If you're on the fence, just get a few songs, and spend your money on something else.