This is something only you can answer.
I think for those on existing systems, this question would be a no-brainer. You're already used to your system's speed, an SSD would be the perfect boost needed to lengthen its life and give you a fresh experience. Waiting wouldn't make much sense, as the impact to you wouldn't be all that different with a faster drive (your old system would hold you back).
On the other hand, with a brand new system, I think an SSD upgrade (especially current ones) would give you a good sense of speed you're looking for - but timed so close to your fresh & fast rig, its impact may be lost amongst the 'noise' as it were. I think you may appreciate it less if you got it now. Of course that all depends on what you intend to do with it, how critical that is to you, and how willing you are parting with your cash.
For example, I've chosen to wait because I went over budget with my system, and I don't absolutely need it right now.