Theo Valich published an analysis of GDDR5 memory, he talks about how this memory standard was formed, its features and advantages, and what we can expect from it in the near-future. You can read his article over here. Here's a snip from his conclusion:
GDDR5 ramped up during 2008 and we expect the technology becoming a standard for GPU add-in-boards in 2009. ATI will migrate to GDDR5, so will Nvidia. With Intel joining the pack with Larrabee, volumes should be ready to drive the cost of GDDR5 into budget for next generation of game consoles, starting in the 2010-11 timeframe.
This is by far the most developed and well-thought memory standard that lacks childhood sicknesses like DDR2 and DDR3. GDDR5 is coming to market as a complete product, and offers solid future roadmap, with Differential GDDR5 even surpassing XDR2 DRAM in quest for highest possible per-pin bandwidth. By that time, Differential GDDR5 should be cheaper than GDDR3 is today.