1) Power full companies - Don't sue them,Work with them.
If you sue, they may counter sue and take off your products from market. Finally you end up in protecting your resources and the damage created by them.
ex: Sun counter sued NetApp. Now NetApp is saying its customers - "we are not closed"
2) Equal power companies - Don't sue them, work with them, exchange IP so that both can expand.
ex: Cisco and Apple on Iphone trademark.
Apple and Sun on ZFS.
3) Low power companies - SUE them ( ex: Vonage - they will pay you all lifetime )