The Tonestyler offers a different sound than a traditional tone control. It is one where the mids are not lost. I have found my P bass to be more present in the mix while retaining it's place in the appropriate register. It may sound honky soloed in some of the settings, but in a dense mix it is very musical.
It is a simple device that does a lot of things well, but it doesn't behave like a traditional tone control. If you are used to a traditional tone control, it will require you to rethink what is possible. Some bassists (fine ones I'm sure) are content with traditional technology. If you are looking for more options, and some very good ones, then the Tonestyler is worth the effort to explore. It is a powerful and versatile tool, but it is not the same as the trusty traditional tone control, and will not give the same results.
One of the gems I have mined is that with a lower frequency setting, the present mids allow for a deep lows that are not lost in the mix. In something like a blue grass influenced rock song, it is huge and drives the rhythm with beefy authority. You could get something like this with an active on-board tone circuit, but you won't get rid of the electronic sound that on-board pres also introduce.