Mike Edwards (at SUGCON 2015) did an awesome presentation on data modelling (I would highly recommend you check it out if he re-runs it) on the pros and cons of how to model data. Glass Mapper (unlike wrapping frameworks – Synthesis / Fortis) is uses the Data Mapper pattern to represent data. He also wrote a great post here – http://www.glass.lu/Blog/TemplateVsRendering
This gives distinct advantages in the functionality it can provide in terms of its exposure of native .net types, its ability to be able to read / write between databases, control of serialization, non-reliance on code generation, mocking, testing, caching etc.
It does also have the downside of being heavier on instantiation in comparison to simply wrapping the item and performing casts on demand. It is also further away from the originating record. This also means that unlike wrapping frameworks, the number of mapped properties will actually also impact the overall performance.