I booked a room at St Athans because there was a railway and tube strike planned the next day and I wanted to stay overnight at a place within a walkable distance from St Pancras in order to catch my Eurostar on time.
If that is your purpose, I would highly recommend it as the location is fab, 10mins walk to Euston and St Pancras station.
I only stayed one night in room 9 (third floor), I arrived at 10PM and left at 11AM. Check-in and out went smoothly. Friendly staff. The room was very clean. Cleanliness in shared bathrooms was OK. It's just a detail but an easy one to provide: I missed a hook or bench to put my clothes on while showering.
It's worth mentioning that I had a £40 Airbnb coupon so I only paid £60 which would otherwise have been +£100 on a weeknight. Knowing that London is insanely expensive, I guess the value for money is fine but that's still a lot for one person. I probably would've booked a bed in a dorm if it wasn't for the coupon.
Overall good.