If money's tight and you could use the money now elsewhere, wait.
If money is not an issue, buy it now so that you don't forget and miss the deadline by 1 day and have your computer break on the 2nd day after your warranty expires.
If you're asking whether or not it's worth it - it most certainly is. I've completely replaced the hardware on my laptop (new HDD, new keyboard, new mobo, new screen). I've also replaced the power supply on my current iMac and imagine there will be something else to replace in the next 2-3 years.