The best replacement for lifesize is polycom.

The best alternatives to lifesize are rated as following:
★★★★★  polycom
★★★  tandberg
★★★  vidyo
★★  radvision
★  cisco telepresence
  scopia
  lifesized

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Speaking about lifesize, one probably shouldn't forget about lync.

Other search terms related to lifesize and their ratings are:
★★★★★ lync
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Google Trends

Scale is based on the average worldwide traffic of lifesize in all years. Click on graph for more.
lifesize
polycom
tandberg