The AF sux, but other than that, it's fine.
Note, I use a really old Metz 60CT-2 flash that doesn't support pTTL, so I'm not involved in that bit of hell on earth.
I think it makes my life easier, actually.
I also don't use zoom lenses, which also makes my life easier.
I've been second shooter with both Nikon and Pentax gear to help out on a few occasions (to put it simply, it's not my day job). I haven't struggled much more with the Pentax gear because weddings usually aren't sports events and I've found the illuminator on the flash helps with focusing speed. Anticipating also helps a lot. It's a little easier with the Nikon gear (flash exposure is more constant than with Pentax and Canon for example) but you still need to get your technique right.
A week ago, Suse B posted a wedding she shot.
The Phoblographer also brought a K-5 with his D3 to shoot a wedding. Overall, he liked it.
Pentax works pretty well for weddings.
I never use AF-S. It's always AF-C. I also use the 540FGZ flash. Main lenses are Sigma 50-150 f/2.8 HSM and Sigma 18-50 f/2.8. Mainly, the Sigma 50-150 though, about 80%. I can shoot an entire event with that lens. I'm not much of a wide angle fan. I have the Pentax 50-135 SDM lens but the AF is too slow for me.
If you're like me and you use a lot of flash and a Quantum type battery, the only problem you'll have is the flash possibly smoking after heavy duty firing. I have to keep that in mind sometimes. If I'm firing the flash really heavy duty, I can smell the burning plastic as the flash over heats. The front diffuser panel on my 540FGZ has burn marks. I though it was just dirt at first but when it wasn't going away, I noticed that it was actually eating away at some of the panel.
With any DSLR shooting a wedding, you're likely to have some out of focus shots. It happens. I haven't missed many critical shots because of Pentax focusing issues as others have.