I'm in the middle of fitting an IKEA kitchen. I got the carcases from
them, as well as the doors, but sourced worktops and everything else
from other (online) suppliers.
IKEA units are pretty quick to assemble, and the doors I'm using look
very good (Tidaholm range).

One nice thing about IKEA (may apply to all the DIY shed ranges) is
that if wifey changes her mind about what she wants inside the
carcases you can just buy the bits you need and replace them.

If she changes her mind before you have unpacked the flat-packs you
can get a full refund on the returns. (If unpacked but not fitted
there is a 70% refund).

Downside to IKEA carcases is that the backs of the cupboards are at
the back of the carcase (normally the back of the cupboard is inset a
centimetre or two to allow adjustment against a wobbly wall). This can
give problems.

Another problem is that IKEA carcases are deeper than others, so a
600mm worktop is flush with the cabinet fronts once the doors are on.
I am running the splashback tiles below te edge of the worktop to give
me an extra cm to counteract this.