I've always been in favor of investing in things that I think will benefit me long term, but all investments cost something - time or money usually, both hard to come by. I remember doing an fill-in presentation at a SQLSaturday where we talked about professional development, one of the attendees was pretty frustrated with me when I said that employers can't/won't provide you all the training you need. Body language screaming 'that's not fair!'. We could argue that, but I think better to just focus on what the world is....it is what it is.
So if you can make the leap to decide to invest time/money, the next part is how/when/where?
Dan Appleman had a article in Visual Studio Magazine titled Show Me the Money that talks about the importance of deciding which technology to learn. If you look at our own history in SQL Server, are you a happy investor if you spent a lot of time on English Query or Notification Services? Will learning Reporting Services help you get/keep a job, or is a distraction compared to learning more XML?
There are no easy answers. Early adopters sometimes get rich, sometimes get the hatchet when the technology fades away. Look at the current job ads, can you see many ads that match your skills? What could you add that might separate you from the pack? Would some MySQL/Oracle/Something Else experience help, or should it be something else within SQL Server?
Decide to invest first, then think hard about how much and where to make the investment. Don't look for immediate returns, and realize that not every investment will have a big payoff.