An interesting game indeed.
Airborne marines are a strong force, but need to be used carefully balancing out the benefits of the air assault against the risk of losing the formations to a counter-attack. However, it is usually better to keep up the pressure - though this does require some advanced planning to ensure the rest of the army can back up the assault, otherwise you risk refighting 'Market Garden' and Arnhem
As to the Eldar / Biel Tan, they are indeed powerfull if you are unable to exploit their weaknesses. However I disagree that they are overpowered. The E-UK database indicates that they are winning and losing similar numbers of games against the core armies and this is true overall when including all the more recent lists, the main exception being the Orks. While there are differences between the Net-EA and E-UK lists, I do not believe these would impact these statistics that greatly.