Interesting news reported in the linked article from The Verge:
T-Mobile will limit data speeds on the Apple Watch Series 3 to a maximum of 512kbps despite the smartwatch’s support for LTE connectivity, a spokesperson has confirmed to The Verge. As MacRumors noticed earlier today, the throttled speeds are mentioned on the carrier’s website. Apple isn’t yet advising customers of T-Mobile’s capped data performance. The 512kbps limit is also applied to smartwatches from other companies such as Samsung.
In the end I'm not sure that the slower data connectivity on T-Mobile is really going to matter all that much to the end user, but it is something worth considering if you plan on buying the Series 3 and are a T-Mobile customer...
The original article from The Verge can be found here
UPDATE: T-Mobile customers reacted to this news and T-Mobile has responded by changing their $10 plan to have unlimited LTE data. Thanks @BillGillilandJr for the heads-up!