Fantastic or not, that's up to whether you like longer games in this style. This game can last 2 hours, but it can be quite a lot of fun for many. If your group likes skirmish/action games you should be fine.
SDE1 and FK (as well as all expansions and warbands) are compatible* with the new FK rules (both Classic and Arcade), which you can download from Soda Pop's website. You can also try the Beta 1.3 rules which you can find on Soda Pop's community forums. Beta 1.4 is coming this week, with initial fixes and improvements by Justin Gibbs (of Malifaux fame)! However do take note Beta rules are written with only Classic mode (1vAll) in mind for now as it is the "main" mode for testing and balancing.
Also, if you love their sculps, check out the SDE:Legends Kickstarter. There is still time for a late pledge. =P
The only thing you miss out from getting one over another are the minis. The bosses and mini-bosses mostly have unique play styles and abilities, and simply look cool, so there are many (addicted) collectors as a result.
Edit: I think it is important to add that there are heroes and bosses that are stronger than others and can break the game for some with the current rules (FK and even Beta 1.3). However some groups don't mind unbalanced characters and that can still be fun!
Edit 2: *Thanks to /u/bobbothegobbo for the reminder that everything prior to FK requires the upgrade (Amazon Link for example) for them to be compatible.
I think people have answered most of your questions, they are revising arcade mode as we speak and there are updated versions on the web, basically they agreed that there is something wrong with arcade, and want to fix it.
FWIW Descent 2 has an app that will GM the game for you so you can solo it, or just play he heroes. iOS / Android