Yes, plenty of people believe 5e is coming - I think they're wrong. The speculated anniversary edition makes more sense for me, but purely for selfish reasons. I might pick up a cleaned up version of D&D that works with older materials, but I wouldn't buy a 5e unless its a huge improvement on 4e - and I'm really happy with 4e.
But to the OP, what you've got to decide is: is the money for DDi worth it to you, and is your campaign likely to continue longer than say 1.5 - 2 years (the earliest most people think 5e could really come out).
I subscribed to DDi when the online magazines and compendium were all I could use as a Mac owner. For me it was worth it, as the magazine subscriptions of the dead tree versions for a year was about the same, and the compendium was enough for me to run a game. Now that the character builder works on my machine without any hassles, and is almost good enough for my needs, I'm still satisfied - even if the magazine quality has gone down.
Edit: removed a rumor since I can't substantiate who the author was.