The items that are worth it are the items that you are going to use.
If you're into running, buy a Garmin. It is hands down one of my favorite things in the entire world. Many people swear by the Forerunner 410, but I love my discontinued Forerunner 305. I recommend purchasing the Forerunner 310XT because, like the 305, you can also use it for biking. It's the much sicker version that syncs your data wirelessly and can be used when swimming.
If I leave my Garmin at home or go for an impromptu run, I use the Garmin Fit app. It kills your phone battery and I've managed to accidentally pause it multiple times, but if you're used to using Nike+ to track your runs, this is a superior app.
A good running jacket is essential because it's miserable to run in a jacket that is too bulky and not ventilated. I wear the Brooks Silver Bullet.
Never skimp on your running shoes. You should be properly fitted at a running store where they can analyze your gait. At most places this is free.
If you're not into spinning regularly, don't waste your money on spin shoes. However, if you go at least once a week, they're a great investment because they really change the workout.
Tech materials have really changed my life, but you don't need the expensive brands. The Frank Shorter line offered at National Running Center is much more affordable that lululemon or Sugoi, but I've heard its just as good.
I generally try to follow the rule that if I am going to use it all the time, then I want the best that I can afford. If it's something I'll use only a few times a year, then it's not a priority for me and I will try to do without.
That being said, my dream in life is to wear head-to-toe lululemon and go to back-to-back SoulCycle classes like all of the women on the Upper Eastside who leave their children with nannies and congregate on 83rd Street.