I'm still playing my penny whistles. Because I like to play like six instruments and various genres/repertoires, I have to spread my music practice between them.. so I don't progress as fast as i could if I devoted my full attention to one instrument and one kind of music. Alas!
But even though I don't play a whole lot on the penny whistle, I do find I'm still sloooowly improving.. yay, I'll take it!
I'm amazed at how different each whistle brand and key sounds and feels to play. I now understand why some folks buy many whistles to try out... especially since it's so much cheaper than it would be 'experimenting' with guitars or mandolins for example.
I now have over a dozen whistles... but there are at least 4 that I don't care for and will sell soon... and 3 or 4 that I want to buy and try out in the future. lol
It does take a while to find the kind of whistles that 'suit you'... in size, playing characteristics, and tone. For instance most Irish trad players tend to like a 'chiffy' tone with some breath texture to it. Personally, since I play more medieval or English country dance repertoire, I myself tend to favor the whistles that sound more 'woody' or flute/recorder-like. The polymer or wood whistles usually sound more this way, while the metal whistles innately have more 'chiff'. But I have a couple of traditional brass whistles that have a very appealing tone appropriate for Irish tunes, that I like to play very much too.
Penny whistles are less expensive than dulcimers by far, so it's easy to have a collection!