The bit between the M62 and the 3Nuns roundabout can be busy, but they're looking at putting a Sustrans cycle route in. From there on there's a cycle lane most of the way through. I use it nearly every Sat/Sun as the cycling club I'm with (Ravensthorpe CC) is based in Mirfield.
Also, there's cycle lanes from near the A62 junction right through to Dewsbury - but I don't bother using them.
It's a bit hairy in places, especially the section from the Three Nuns to the motorway junction. I regularly rode Bradley to Dewsbury, and there is a Tarmac gSustrans greenway to keep you off the main road most of the way. The road is busy through Mirfield and Ravensthorpe at rush hour. There is also a shared use pavement along by the Three Nuns.
Cheers both, I'm planning another exploratory ride on that road to see how it is.
Just a bit of info re the cycle route DC mentioned - Sustrans have the £££ allocated to Calderdale Council to link the Calder Valley Greenway and Spen Valley Greenways - this would link a
lot of towns together on relatively flat (for the area lol!) routes but there is a sticking point with a landowner by Cooper Bridge where it crosses Calder/Hebble navigation.
There some info online about it, I'll dig it out tomorrow but it appeared on one the local rags websites in last 6-8 weeks, I also spoke to xxxx of Calderdale Council's cycling/sustainable transport group and he confirmed the same - there is £££ to link the routes, but landowners are being awkward.