It's how many used to be, before the arrival of SIG and ZIG tents. They offer the bathtub benefits so are better than a flat groundsheet, but won't offer the sealed-in draft-proofing of a SIG or ZIG.
Like any separate groundsheet, they're easier to pack up and stow away if muddy. A tent using this groundsheet system takes a bit longer to pitch though and is a lot more fiddly.
Personally, at this stage, I'd only go for one if the price was right. I think most people want a sewn in groundsheet, nowadays.
There's a thread on here about bargain Vango tents which have this type of groundsheet (probably why they are such a good price).
Our first family tent didn't have an SiG. It lasted one season.
It turned chilly and the wind whipping in made the tent an unbearable place to be.
A new tent with an SiG in much worse conditions and the tent is a lovely camp where you are protected from the elements, and sitting in it during a storm can be strangely therapeutic.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.