[asyvan]

In general, for a plug-in hybrid to be viable in terms of cost and environment you need to be able to run it on full electric for a major part (preferably all) of your daily driving.
So you need to consider your one-way daily distance (to/from work for example) and see how that fits with the electric distance the vehicle is capable of.
At work you can recharge the battery and then head home again in the afternoon running fully electric.

When you make longer trips on weekends or holidays you have the petrol engine to give you the range.
The petrol engine will also assist you in full electric mode if you need the extra power for an overtake or something similar.