Pro-ject speedbox: Is improvement worth the $ ?

They usually make a very nice difference in sound quality. But it depends on what you have your TT plugged into. The reason speed controls usually make a big improvement is that speed will fluctuate with your AC. Its like a light bulb. You can sometimes see it get brighter or dim at times when the AC varies. The speed of your TT is subject to the same types of variations. If you have a line conditioner that regenerates your AC, you may not need a speed control. A good regenerator will keep your AC at the same level all the time, so if you have your TT plugged into one of those, its probably not necessary to get a separate speed control. Some PS Audio regenerators, like the 500 are known for being used as a dual purpose TT speed control's, as well.

There is also the recently introduced option of the Phoenix Falcon speed controller, to which a Roadrunner Tachnometer can later be added for even greater speed precision. Cool stuff.

The Speed Box only provides the user to change speeds at the push of a
button. It does not have the ability to fine tune the speed to compensate for
deviations in pulley, platter, and belt tolerance.

Since your motor is an AC synchronous motor, it is not dependent on stable
ac voltage like a dc motor but relies on the grid's 60 hz frequency.
Fortunately in the US, the 60hz frequency is highly regulated and controlled
and does not deviate like line voltage. A true motor controller will have the
ability to change the frequency thus allowing a fine tune of the motor's
speed.

I wouldn't purchase a Speed Box unless I frequently changed between 33
and 45 records. Spend your money on a real motor controller, not a switch
box.