Disclaimer: I used to go to a church that Lawhead used to attend (St Aldates, Oxford) and I've met his son Ross a couple of times, so I may be biased. That should also cast some light on where I'm coming regarding the Christian aspects of his writing.
I've read a fair amount of his stuff. When I was younger The Pendragon Cycle was one of my favourites. As time has gone on I've grown to be somewhat less impressed. Now, I find that my thoughts are so caught up in nostalgia that I'm not sure how "objective" an opinion I can give. I guess I'd say that I think Taliesin and Merlin are still fantastic, but the latter three aren't quite so good. I don't think the Christian aspect is cloying in the slightest, but if you're more sensitive to that kind of stuff then you may find it off-putting. (If you're writing an Arthur tale set in Britain in the Dark Ages then Christianity's going to be there somewhere, and it's only a small jump to then make Arthur part of it.) But there's no denying that Lawhead's writing with an agenda.
The books of the Song of Albion trilogy are fun, and I've enjoyed reading through them two or three times, but they're not anything really special.
I haven't read much stuff he's published since 2000 - that discounts the Celtic Crusades and the King Raven trilogy, as well as his current Bright Empires stuff.
I think my favourite work of his may actually be the science-fiction Empyrion Saga which I've only ever come across in a single volume, although I think it's also been published as two books. It's a tale of wormholes and time dilation, and a planet with two radically different settlements. It's got possibly the strongest Christian bent, but that's never been a problem for me, being a believer myself.
I'd always recommend them as being worth a read...but probably not at the top of my list!