Does anyone have any experience (good or bad) with those inexpensive
plastic
walk-in greenhouses, the type that are sort of like a tent that you can
get
at Argos or Index, etc?

No experience, but I bought one of the 4-shelf ones (not walk-in)

I've got one of these I bought from some DIY shed. It has lasted well for
several years (and put up with a lot of bad treatment).
Hayley