Don't get me wrong, I love Yahoo Mail. I started with it in 1995 or whenever it was it first kicked off, and I still use it almost religiously for my personal messages.
I've been checking it on mobile devices since 2000 - initially on a browser on a Palm PDA, hooked up via infrared to a Siemens SL45i phone connected over GSM. More usually, I use either a handset's WAP browser, or a normal HTML browser on a smartphone.
Recently, Yahoo has messed up, though. A lot.
Firstly I played with it's Yahoo Go software on the trial N73 I had running on 3's X-series service. It was utterly, utterly dreadful, the single worst piece of mobile software I'd ever used. Slow, temperamental, didn't have a 'refresh' button so I couldn't see new messages immediately, limited memory. Just ugh. Worse than the 2000-era Palm/Siemens/infrared user experience. And much worse than just using a good HTML browser set for full web pages, downloaded over 3G on a Nokia E60 I was also using at the same time.
And now it seems to be trying even harder to mess up. I've got a Windows Mobile HTC device that I use for web browsing on a T-Mobile Web'n'Walk plan. Historically I've done 'pull email', checking Yahoo a few times a day. Worked fine. But now they've gone an 'upgraded' (!) the site to its new Mail Beta. Yes it's a bit prettier, but takes longer to load in, and stupidly only displays 5 messages per page, with no obvious option to let the user change it to 10/20/50 etc. Hello? Yahoo? Tell me - what's quicker, scrolling down... or going back for another roundtrip to the network? Let me see, outside of 3G coverage on T-Mo's GPRS, I reckon that's 0.3 seconds for option 1, versus 7+ seconds for option 2. And at least up until last week you still had Mail (classic) as an option on the main Y! menu, but you've now removed it. Put it back, NOW.
UPDATE: I've noticed something even worse about Yahoo's mobile beta email. It only loads in a single page of text from messages, requiring the user to click 'more' after about 10-15 lines depending on your screen size. This is outrageously stupid, and obviously straight out of some 2003-era book on designing a mobile user experience. Guys: phones have scrolling functions! People use them regularly, eg for scrolling through their contacts lists or message folders. This isn't rocket science.
Yahoo - I'm a (very) loyal user, so please, please take your mobile mail strategy back to the drawing board before I have to switch to someone else.