I would like to order custom frames from an online supplier and they come with 1mm acrylic (plexi) instead of glass. The frames will be 16"x20" or larger. Is the 1mm acrylic a good choice here? Some of my unknowns are is it rigid, sooth and completely transparent? Will it bow or show uneven reflections, etc. Basically, is a suitable choice for framing of photos this size? This particular glass has no UV protection or anti glare. It's a standard, entry-level product. I would really appreciate actual experiences with this material.
Acrylic that thin is fine for small prints (11x14" or smaller) and tolerable for larger ones if you watch out for a couple of things:
Thin acrylic isn't particularly rigid, which means the front of your print gets less support than it will with glass. This gets worse as the print gets bigger, and thicker material will bring some of the rigidity back. You can make up for some of it with a sufficiently-thick (and stable) backing and enough of a lip up front to hold everything securely in place. Inexpensive frames come up short on this, and support for the print at the edges and corners will wane over time unless you have a whole lot of clips along the inside perimeter. Having your lab mount your prints on styrene will help with this, too.
If you intend for these frames to last awhile, I would recommend treating the outward-facing surface with a plastic cleaner and protectant such as Plexus or Novus Plastic Clean and Shine. These will help the surface resist dust and smudges, cutting down on the need for cleaning and opportunities for scratching.
Once hanging on the wall, you can't really tell it from glass without touching it or getting up close enough to see how far back the print is from the surface. I have a couple of posters in frames with acrylic that have been hanging for 20 years. The material has done fine, but I have had the support problems mentioned above.