The phrase "a keen user" is correct and usable in written English.
It can be used to describe someone who is enthusiastic or highly skilled in using a particular tool, software, or technology.
Example: "As a keen user of the latest graphic design software, she was able to create stunning visuals for the campaign."
Alternatives: "an avid user" or "a passionate user."
She is also a keen user of new technologies such as Twitter and Facebook.
Earlier Musharraf, a keen user of social media, took to Facebook to decry the charges against him.
Lauren is a keen user of Twitter for science communication and can be found at the handle @laurencowley4.
Stephen was a keen user of Twitter which said that 11,000 tweets had been sent with the hashtag #ThumbsUpForStephen.
Polonsky challenged Lebedev to a Twitter duel (Lebedev is a keen user of Twitter, and has a website called the "Alexander Lebedev information channel").
At Crosshall junior school in Cambridgeshire, deputy head Anne Eardley is a keen user of Apple's iLife - "a brilliant package in which every element speaks neatly to everything else and enables children to produce some remarkable results in print and film".
It isn't bricking any devices or introducing serious new bugs, but I would be cautious if you are a keen Apple Health user since there are just enough complaints from users losing their data after upgrading to suggest you shouldn't risk it.
Quentin Blake's Mrs Armitage is a keen bike user.
Merkel is known to be a keen mobile user and has been nicknamed "die Handy-Kanzlerin" ("Handy" being the German word for mobile phone).
Announced on 30 September 2014, Windows 10 has only been released as a test version for keen users to try.
(Another report out in March will look in to this in more detail).Previous arguments for HS2 have emphasised the need for more capacity on the overloaded West Coast Main Line (WCML) and suggested that businessmen, whose time is valued more highly than others', would be particularly keen users of a whizzy new train.
