I've cycled most of it bar the very beginning Southport>Aintree, its a massively mixed bag of surfaces, environments and effort. I do it on a tourer style no suspension, I've used 28c tyres but prefer 32c or bigger. The main climb, the pennines themselves, is easier West>East as it rises slowly out of manchester before a last effort to the top, East>West its flat almost to the foot of the hill then a big long steep climb but at least that is on road. It has the some of the best and worst scenery of the cross country trails, and some parts (Goole>Hull) are outright dull.
Its worth doing it at least once I'd say, but given a choice I'd chose a different route for a cross country trip.
Top tip would be if you use mudguards adjust them to get extra clearance, and be prepared to stop every so often and get a tyre leaver out to get all the crap out thats starting to jam under them!