You're in for a treat, I've done a few of the Sustrans routes and this was by far the most rewarding, it was also the toughest. There's that many variations it'd be hard to make many specific recommendations. We did it from Holyhead to Cardiff, the Taff Trial towards the end was much like any other cycle track, though by then we were glad of some easy riding. The rest was really scenic. Our favourite B&B was the Horseshoe in Rhayder and the only disappointment Breacon. Nick Cotton's guidebook, that was available from Sustrans, is very useful, in the planning stages as well as on the ride, it gives an accurate description of what to expect on each section. If you haven't ordered it you're welcome to borrow mine. Have a good time.
I did it the other way around. I reckon the hills are longer, but a bit shallower that way. But I could be completely mistaken. I stopped at Bangor and got the train home because I started another ride from Holyhead the year before so I reckoned I'd already done that bit. None of this is much use to you. I'm just posting to say enjoy it. It's a good ride.
Put nice new brake pads on before is all I can say.
If camping, be aware that some camp sites marked on the maps either no longer exist or are (really annoyingly) Caravan Club - caravans only, despite there being a tent symbol. We got caught out a couple of times at the end of a long day! It is not really the mapmakers fault as they are using OS map data which may not be current. On subsequent tours I have done a thorough web search for camp sites and marked them on the map myself.
Take plenty of good wet chain lube - if its rainy your chains will need regular attention, especially with the off-road bits.
It's an excellent route. I started from Chepstow and going north it is really well signposted. It is quite tough in places, the hills tend to be short but sharp and I was glad of a really low gear. Gospel Pass, Hay on Wye, the moors above Rhayader and especially the hills above Harlech are spot on. I can second the Horseshoe in Rhayader and the other B and B's I got from the CTC book of the time were all good. Get the Nick Cotton book and the Sustrans maps and enjoy.
Thanks for your comments everyone we attempted the first half of the route at the weekend ie from Cardiff - Machynlleth. As you might have noticed the weather was terrible! The first day to Glasbury was OK, but the second was a write off because of the ridiculous wind. Instead we turned east at Builth Wells and ended up in Leominster via Kington on the back roads (and a couple of mountain passes etc), to get the train back from there. We still had a great ride and my legs aren't thanking me for it today! Hopefully we'll get to do the rest of it next year.
You will love it as long as you like hills. I live and was born in Wales. I have cycled every bit of it, and I cycled lands End to John O'Groats when I was 58, I have cycled coast to coast, and all the national parks. I am now nearing 70,and have three bikes,and enjoy cycling every day weather permitting.
Take a look at my web site, I have a section on Cycle Touring, and many links to Cycling, including the C.T.C. Sutrans, recumbents, folding bikes etc. http://pawild.net/alanherper