We've been through our whole house a couple of times now. Wallpapering and painting every room. We love doing it when we have plenty of time and space to do it. Hate it when we're rushed.
Over the years, there's been a couple of times we've had "professionals" in, both when our DS was young and we didn't have the time/inclination to DIY. We're weren't happy either time. Despite being "professional", i.e. City & Guilds, experienced, blah-blah, the quality of both was far below what we could do ourselves. Once was the lounge, which we ended up doing ourselves a year or two later because the dodgy wallpaper joints stood out too much. The other was our hall, stairs and landing. We did it ourselves first time but it stressed us having to buy platforms and then took ages to do the longest floor to ceiling 2 storey drops. So got a "professional" for the second time. What a bloody nightmare - the papering was like a toddler would do - not even properly vertical - he clearly didn't bother with a plumb-line. So, again, a couple of years later, we did it ourselves. I think the so-called "professionals" try to do things far too quickly and end up with poor quality finish.