Yep, it was totally worth switching to this mouse.
For starters, the tracking isn't weird like on the M705. It is dead on... I feel like the cursor is just an extension of my hand...
Next, the larger profile, added weight and slightly more aggressive lean angle on the mouse make it a more comfortable fit.
The scroll wheel has the same two-mode feature - either click-scroll or free-wheeling. However, on the MX Master the button you press to switch modes is more flush to the body of the mouse, and only requires a slight click, whereas on the M705 I felt I had to stretch my finger up to press the button in - it just didn't feel easy. Secondly on the scroll whell, the scroll button (click the scroll button for middle click) is a lot easier than on the M7o5 - which again, felt more spongy and required more action. It also meant it was difficult to use middle click without accidentally scrolling.
It has an inbuilt battery with USB charging cable, so no more replacing batteries! I'm not sure what the battery life is like yet.
The only thing I found slightly better about the M705 is the thumb-wing button. It was a lot 'clickier' on the M705, a bit spongy and hard to find on the MX Master. SetPoint / Logitech Options seems to provide good options for changing the function.
The MX Master also has some cool horizontal scroll wheels next to the back/forward buttons on the mouse (which I haven't got working yet). It does make the back/forward buttons smaller and a little harder to use though.
Lastly, the MX Master has both Bluetooth and Unifying Receiver compatibility. A kick-ass feature for me as it can connect to both my tablet (Bluetooth) or my laptop (Receiver). I use Logitech gear where possible because of the Unifying Receiver, but this makes it even better for when I've lost the stupid little dongle or don't have any spare full-sized USB ports!
All in all, I would 100% recommend you buy this mouse over the M705 (or even the M905). The MX Master was about 50% more expensive, but I'm using it every day, all day... totally worth the extra money.