Milgard is a quality product as I have been told (not available in my area ever). The other 2 I have not heard or but just from spending 10 seconds on their sites I would keep looking.

We are installing a 15 foot double sliding Milgard door to our covered patio tomorrow morning. I have been to the Milgard showroom and think that they have excellent products, espectially their sliding vinyl doors. Their Ultra brand is top of the line fiberglass but we didn't go with that because we don't need such a heavy duty product due to the covered patio. We are going with the Tuscany series instead.
Milgard company started in our area and everyone around here carries them. We installed Milgard windows throughout our entire previous home 23 years ago and had no problems with them. They are also installed throughout our home now, probably installed back in 1992 and don't have any problems with them. They have a great warranty. I am sure you would be happy with them. I do not work for them, just have had lots experience with them. Can't wait for my new door tomorrow. Good luck with your project!

Thanks to both of you. I will definitely look into the Milgard Tuscany series.

Are Masonite doors worth considering at all?

Not in my opinion and I do not believe they make a sliding patio door anyway.