Well, not me but my employer has the funds to allow me and my staff to buy multiple SD cards for a range of mobile devices. This is part of our day-to-day business as we are eg. conducting tests with mobile phones, PDAs, UMPCs, smartphones and running VPN clients, activesync push and so on.

From the day-to-day perspective, I mostly don't see a difference whatever SD card I use. That said, we usually buy "our trusted brands", i.e. from brands we know for years now and that (to date at least ) never failed us: Kingston, Transcend, SanDisk (random order). With SDHC we also had to try some new card readers (SanDisk, Lian Li, Kingston...).

We use all different kinds of SD cards: microSD, miniSD, SD, SDHC (micro, mini, ...), capacity ranging from 512 MB to 8 GB with 16 GB following shortly. And every single card holds the specified capacity.
Honestly, although I do prefer class 6 SDHC cards the larger the better I only can tell the difference when transferring serious amounts of data. Just using the cards with a mobile device, all we bought to date are fit for the job. At least that's our experience

Bottom line? I don't worry too much as IMHO I would have to spend 80-90% of my time to squeeze out the last 10-20% of performance. But maybe that's just me

(Side note: As I travel a lot and we are field testing the devices, I have my music with me - that's the main reason I prefer SDHC class 6 )