Hot, yes. Usually quite comfortable though. Though both depend on the design. And there are cooling options out there; small airflow systems for the heads, and cooling vests that yhe other commentor posted a link to.

I find that Stevia has a vaguely creamy flavour to it. Which works well in some instances, and not in others.

Aspartame just tastes awful, for me I get this weird sticky/bitter sensation over the roof of my mouth and turn my throat.

Splenda/Sucralose tastes fine, but has noticeable effects elsewhere, which are a bit TMI.