I also use the Ameda Purely Yours. Sometimes I like it, other times not. Recently I can pump about 2-3 oz on each side in about 15 minutes in a good session and 1-2 oz each side in a bad session. The white valves definitely have to be replaced occasionally, but I'm not sure about anything else. I used to be able to get 4+ oz on each side in the morning and 2-3 oz each side in the afternoon with this pump, but it's been less since my DD turned 6 mo. I've been hoping replacing the valves would help increase the output, but only on some days does it work better, unfortunately. Though sometimes replacing the valves have made a big difference, so I'm not sure why recently it hasn't been helping as much as I'd like it too. Maybe I'll switch out the diaphragms also and see if that helps.
I've been using this pump now 2x a day for usually 5 days a week since DD was 7 weeks old, so I'd say it's generally a good pump, but maybe just for the first few months?