Fast, because of GPU speed. somehow limited to vram of your GPU. I think when you work with scenes that frequently go out of core, the speedadvantage over CPU rendering shrinks significantly? I heard about different experiences here. Maybe someone can bring up more details. no pointrendering at the moment.(advantage of octane). smart biased renderer, advantage over other GPU solutions. best Houdini integration (GPU)
due to the fact that mantra is a sidefx product it will always have advantages in terms of integration when you work with H. handles polysoups and packedprims a.s.o. (redshift is working on this), advanced shaderbuilding with VEX and VOPs, you can not compare the level of flexibility you get here. gridmarkets, amazon and some other online services to expand your render capacity, i don't know any solutions for redshift and H so far. mantra handles volumes and particles better. unlimited rendertokens with one FX license!! learning mantra might give you a deeper understanding about rendering, like working with houdini gives you deeper insights in 3D in general.
I am shure sidefx will push rendering with mantra or (something else?) in the near future. If you look at arnold GPU i.e. things seem to change a bit.
I d say it depends on what you wanna do. When you do personal and smaller scale projects the speed advantage of some GPU's is really great. We see more and more studios adapting such a workflow.