C# 14 extension members look useful because they solve a real readability problem. C# developers already use extension methods heavily, but helper code still ends up split between static utility classes, naming conventions, and methods that do not quite sit where you want them. The new syntax should make some of that feel less bolted on.
Microsoft’s C# 14 documentation lists extension members as one of the main features, alongside null-conditional assignment, better Span<T> conversions, lambda parameter modifiers, field-backed properties, partial events and constructors, and more. Extension members are the one I expect many application developers to notice first, because they affect how everyday APIs read.
The obvious use is extension properties and static-style members that make domain code cleaner without forcing inheritance or wrapper types. The danger is also obvious: turning every helper into something that pretends to be part of the original type. If a team uses this everywhere, discoverability and ownership can get worse rather than better.
So I would use extension members as a refactoring tool, not a decoration. If an operation is already conceptually part of how your code uses a type, and the extension makes call sites easier to read, it is probably a good fit. If it hides a service call, a database lookup, or a chunk of business logic, it probably belongs somewhere more explicit.