Your view on that will depend on whether you live in Milton Keynes or one of the stations between Milton Keynes and Watford Junction.
Unless you absolutely leg it across London or are lucky with the Tube, it's still a faster way to get from Bletchley to Clapham Jn than via London (though you can then get to East Croydon faster at certain times of day[1] by changing at Clapham Jn). When it's been canned coming back from East Croydon I've always managed to get 15 minutes worth of Delay Repay going via Vic or Thameslink. You might beat it from MKC with the Avantis only taking 30 minutes, though. And there are advantages of going to Bletchley instead of MKC if you live between the two - plentiful parking and far cheaper, for one, and the whole experience is just a lot less "rat race", having more the character of a wayside halt.
I also love the crossing of the Thames which is almost as scenic as the 59/68/168 bus.
That, and 4 out of the 8 coaches are all tables so you can actually work during the journey, done that a few times.
[1] The general pattern is xx17 off Bletchley, however the 07xx is 0706, presumably for peak WCML pathing reasons, running 11 minutes earlier than normal all the way to CLJ where it then sits for that time to regain the normal path which is clockface all day. So if you leg it over the bridge you can pick up a fast and get there quicker.