The best replacement for m600i is w950i.

The best alternatives to m600i are rated as following:
★★★★★  w950i
★★★★  e61
★★★  p1i
  nokia e61

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Speaking about m600i, one probably shouldn't forget about isync.

Other search terms related to m600i and their ratings are:
★★★★★ isync
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Google Trends

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m600i
w950i
e61