I recently had Pella Impervia fiberglass replacement windows and doors installed in my newly purchased home.  I chose this brand and type because it got very good reviews in Consumer Reports.

The windows are a huge improvement over the 40-year-old aluminum windows that were there before.  I had wanted casement windows, but also preferred fiberglass over wood or vinyl, and casements are not available in the Impervia line.  I went with double-hung windows without panes, for the most modern look I could get.  I am happy with the fiberglass...no upkeep needed really.  Wood windows have to be kept...