Well unlike some of the other posts that try to distract you away from the info you really want with non-objective points such as "money pit", "shiny objects", "still a beta" and the such, let's see if we can actually help you get some info so you can make an objective decision:

Quote - I have Poser 2008.

There's a Poser 2010 and a Poser 8? Could it be one of those?

Quote - I'm not anything close to a master in it, but have created a few characters I like for my project in David 3 and Michael 4. I have a few more characters to create. I need to buy Michael ++ Morphs and Vicky 4 ++ morphs to get more variation and then I noticed the Genesis option.

So you don't have much invested in gen 4 options? If not, you have a pretty clean slate to work with. Do you work primarily with M4/D3?

Quote - Genesis looks pretty easy to use and morphing the characters seems easier. So, I have a three simple questions:

Why should I move to Genesis?

People can give you different reasons for that and it all depends on what you want to do. The big idea idea behind is it is that one figure serves as a base for all figures. So you can derive a male and female, as well and creatures and a gorilla from the same base, all with different scales. Also it uses weightmapping so a good chunk of bending issues have been resolved from the previous generation and you can scale the figure as well. These morphs can also be mixed together to create brand new morphs as well. So you could mix a female and gorrilla together to get a female gorilla if you wanted.

If you like creating people and creatures, then you'll really like it because you can create different morphs and clothes designed for it will fit. Now extreme morphs may not look too good, but the clothes still fit.

Quote - Why should I not?

If Poser is your app of choice, then the figure isn't completely compatible in Poser 2012 and Poser 9. Right now, it works completely in DS4 and somewhat in Poser 9/2012. It doesn't work in lower versions. The exporter that is part of DS4 can export a genesis character to Poser but it has a long way to go for compatibility. The figure will be lower in polys than in ds4 (because DS4 uses subdivision in it's platform) and clothes currently don't work. 

In DS4, the autofit tool mangles shoes and removes morphs from clothing. So right now simple clothing works best until they can fix/add more features to it.

Also with it being a new figure, there's not a lot of content for it yet, like a good pair of shoes. The autofit tools helps with some outfits but it's not perfect.

Quote - If I went with Genesis can my David 3 and Michael 4 characters make a seemless transition to Genesis?

There are clones for both David 3 and Michael 4 for genesis, and the GenX by Dimension3D tool currently allows you to bring over morphs from Michael 4 and he's looking at adding D3 support. Also with the David 3 clone if you have the autofit tool, you can also import clothing for D3 and it will fit genesis... but will have the same issues as importing other clothing. I've also used the content toolkit to convert some head morphs from D3 to genesis.

But really the best thing to do is to try it out in DS4 and see if you like it or not, because I nor others shouldn't take the place of your own use with the figure. But since you sound like you don't have a lot invested in content, you really should do a hands on with both and see what you like. Check out the freebie thread on the DAZ common forums where I did some head morphs for both Gen 4 and Genesis and you can try both out and see what you like.