First, the most important thing to consider is that this is not an upgrade, it's a migration. This is an important difference because this means that your current extensions and template will not work in the new version without some work. Due to the change in the category structure - remember, no more sections and we get nested categories now - there are significant changes to the database tables that house the all important content. There is at least one migration component available, but it's in it's very early stages at this point and there are considerable issues in getting everything to work right. At this point, very few developers have started the process of upsizing their extensions to work with 1.6. In fact, many are waiting to see if the 6 month release schedule is actually going to be met and how that will impact the way extensions are coded in the future. No one wants to do the work twice!
There is also the fact that 1.6 is not going to be as stable as 1.5. The "old" version has had nearly two years to be polished and cleaned up and it is definitely a fine product that still has a lot of life left in it. It is very likely that there will be bugs and security issues found once 1.6 starts seeing heavy duty use in a live environment. Even though extensive testing is done, it's hard to tell what new and innovative ways users will break sites with. I am sure we'll see several patches released before 1.7 hits the streets this summer.
Basically, it's this developers opinion to hurry up and wait. See how the new system plays out and how fast developers jump on the bandwagon. There are already several of the larger developers that are on board and have updated extensions being released in the near future. There are already a few hundred in the JED, but that is a far cry from the several thousand for 1.5 that will need to be worked on. Time will tell, but 1.6 does look very promising. If your site has basic needs and you'd like some access control, then it wouldn't hurt to give 1.6 a try. However, if you are using any extensions and have a site that is running 1.5 already, there is no point in messing with the headache just yet. End of life for 1.5 is still over a year away so there will be plenty of time to plan and get it right the first time.