The short version
Bang Yai is a western suburban centre built around the Purple Line, large retail and extensive housing. It offers more space and a quieter domestic routine than central Bangkok, but it is not a place to choose casually if your work is across the city.
Around Talad Bang Yai and Bang Rak Yai, rail and shopping make daily life relatively legible. Beyond the corridor, gated estates and local roads spread out quickly. The home may be comfortable, while the walk to a station is not. Check feeder transport, drainage and school traffic in person.
The Purple Line connects Bang Yai through Nonthaburi to Tao Poon. That is a meaningful advantage for north western commuters. For everyone else, the journey includes a transfer and the first or last mile. A car helps, but road congestion can erase the apparent convenience.
Verdict
Bang Yai is sensible for families and home based workers who value floor space. It is not a substitute for a central neighbourhood. Choose a station linked address if rail matters, and do not pay for a view of a mall while ignoring the daily road outside.