Yes, I'm on my second set of Mark Glenn Hair Extensions (for length/volume) and they're fantastic, have had no problems with them. The only downsides for me was that I had to buy some different straighteners to the ones I had as they weren't suitable and, of course the extensions from Mark Glenn are more expensive than average (prices recently went up a bit - www.markglenn.com/hair-extension-prices.htm - but they do offer a money-back guarantee if you're not happy).
I've worn extensions for nearly 5 years and have tried most methods. With Mark Glenn, so far, so good (have been wearing for nearly 6 months, appt for third replacement in a couple of weeks).
They're much more comfortable than others I've worn, they look better for longer (although they seem to look better after the first time you wash them rather than straight after they've put them in), they feel lighter and are really easy to style in comparison to human hair. I've had none of the tangling or "dull" appearance that I've always found with even the best quality human hair ones and I've had lots of compliments on my hair. There's no fancy maintenance regime either and I can swim with my hair down without problem.
I do get a bit of a headache the day after they're put in (probably because of all the manipulation with your own hair - no glues) but I've seen no damage to my own hair, in fact it seems to be recovering (I had a lot of breakage from previous hair extensions).
So yes I would definitely recommend them, just make sure you know what you're getting into because extensions can be very addictive! ;-)