Ikea are absolutely brilliant. Some may have issues with their style or perceived quality however for price and flexibility they dominate.
Other companies are trying to mimic the flat pack business model. However the overall shopping environment, forcible furniture, warranties make them standout.
The sofa is just one example of adaptable furniture. One limitation though is their bed and mattress products. They have different dimensions (unique to ikea) and therefore you need an ikea mattress for a perfect fit.