depends what kirby you have. i have a vintage dirt meter for pre g series kirby's and rarely use it except for mini meets. if you have a shake bag kirby, you see the results after each use anyways. they are fun to have, but dont invest too much in it. theyre ohh ahh wears off after a while and they wont get used much.
But then the fun wears off after a few uses. They are useful if you have several different types of vacuums that you want to test out by vacuuming afterwards with a Kirby and dirt meter attached.
You can get the new style dirtmeter for the Kirby G series for between $18 and $35 online.