General application performance can improve a bit by switching to Core 2 Duo, but the biggest performance gains are associated with 3D rendering and media encoding tasks. Considering the nature of the improvements to Intel's Core 2 processor, the areas in which it succeeds are not surprising. If you use your notebook as a professional rendering or encoding workstation with no desktop in sight, then you'll probably consider Core 2 Duo a lot more carefully than most.
One of the items that clearly stands out is that discussing gaming performance on laptops is largely an academic endeavor, as the vast majority of shipping laptops are going to be completely GPU limited. We will hopefully have some results from a high-end gaming laptop in the near future, at which time we can detmine how much of an advantage Core 2 Duo really has over Core Duo. The designs are similar enough that we don't expect a huge difference, and the lower FSB bandwidth will certainly limit performance potential more than on the desktop. However, we would expect a difference somewhere in the range of 5-15% in most games if we can remove the GPU bottleneck as is evidenced by the Oblivion results.
While Core 2 Duo does look nice, as long as you've got a good notebook today you'll probably want to wait until Santa Rosa before upgrading (at the earliest). With Santa Rosa, clock speeds will go up slightly but more importantly we'll get access to a faster FSB. Unfortunately a side-effect of keeping Core 2 Duo fed with a faster FSB is that while performance may go up, battery life may go down. It'll be interesting to see what Intel can pull off with the new platform; one of the funny things about performance and battery life is that if you can complete a task quickly enough thus returning your CPU to an idle state faster, battery life will grow even though instantaneous power consumption may be higher.
For Apple users this means that early adopters of the new MacBook or MacBook Pro won't be too pressured to upgrade again by the end of this year. Of course Apple has this way of making incremental changes irresistible.
Overall, Merom may not be as big of an upgrade to Yonah as Conroe was to NetBurst, but the bottom line is that you get equal or better performance in every test without increasing cost or decreasing battery life. Owners of Core Duo laptops really have no reason to worry about upgrading for now, and waiting for the Santa Rosa platform before your next laptop upgrade seems reasonable. Those looking to purchase a new notebook on the other hand have no reason to avoid Core 2 Duo models, assuming pricing is consistent with what Intel is promising. There will be a delay of at least a few more weeks as we await availability, and testing and validation by laptop manufacturers may delay things a bit more, but within the next month or so you should be able to get a Core 2 laptop.