Flush with cash, I bought a set of coils for my 2008 Prius off e-Bay 6 months ago and got around to installing them a few weeks ago. Everything ran fine until yesterday. When I started the car, the engine shook, the "Check Engine" came on, and it struggled just to reach 50 mph. When I took it to my master mechanic, he hooked up his code reader and said he suspected a coil was bad. I told him I had changed them a few weeks ago. He asked if they were Denso parts. I said "no". I only paid forty bucks for the set. After chasing me around the garage with a torque wrench, he told me to NEVER buy Chinese parts for a Prius (I didn't want to tell him about my Chinese HID light bulbs). So if you're in the market for coils, don't buy ones marked "ECCPP".
Cheap coils have been a little hit or miss for me. For my V6s and V8s, I have tended to favor the cheaper ones just because the savings are multiplied a bit more than in an I4. Every once in a while, a coil will fail, but they're generally easy enough to install and troubleshoot that I don't mind switching one out every now and then. For my Prius, where I'm only buying four coils, I think I'll go with the OEM-recommended coil. Buying non-OEM coils carries a slightly higher risk than OEM, but the price tag reflects it. Sometimes it's worth it.
