S.T.
2 things to consider:
When it's time to replace the power unit (battery) it can be very expensive - it's down the road - but from what I understand the cost is expected to be between $4,000 - $7,000 depending on the model. That's a huge "repair cost" when the car is getting older and you're trying to decide whether to sell or repair. I have to imagine that it would be hard to sell when it's due to have a new power unit.
If you're doing this to be green, research the manufacturing process and location of the battery / power unit - although there is one or two factories in the US that make these power unit most are made in India, Indonesia, etc - where OSHA and environemental laws are nearly non-existent. So while you may be doing this for a good reason, the manufacturing process may be causing much bigger workplace & environmental hazards than we realize.
I've been considering a hybrid but don't feel certain on either of these accounts yet - so my "jury is still out" on hyrbids. Keeping my almost 10 year old car is still more environmentally friendly since the resources needed to manufacutre it 10 years ago hasn't been needed needed again.
I'd be curious to see if any other posts here have answers for these questions.