For me, it was worth it! DH bought me a Go Baby for Christmas and I bought the 4 1/2" square with 2 1/2" square and 1/2 sq triangles die. I've cut almost all of my 1/8 yard strips into squares and am loving:
Knowing that the squares are accurate and my top will not be off point because of a slipped ruler when rotary cutting
Not having the wrist/arm pain I get with rotary cutting
Saving soooo much time... I can cut well over 4+ squares with the Go Baby in the time I could cut one with ruler and rotary
The small footprint of the Go Baby... really it takes up less room (with the die) than my rotary cutting equipment, so easy to tidy up
I don't think it is a money pit. The time saved and accuracy gained are well worth occasionally replacing a mat, IMO. I only anticipate getting one other die so that is no issue for me.
Good luck whatever your choice!