Are you asking for the future or for now? Because I realize that Adobe has discontinued development of Flash for linux, so I assume that, at some point in the future, development will add features that the version available to us will not have - and that websites will be utilizing those features, meaning that we will not be able to view flash content on those websites.
However, it seems to work well enough now, doesn't it?
I just had a quick look at the website you linked to. The current status section does not state that their Flash player has all the capabilities/features that Adobe's Flash does; rather, it only states "Lightspark currently supports YouTube well enough for daily use." Which is an accomplishment, certainly, but it infers that there might be much still to accomplish before their work can reasonably be called an Adobe Flash replacement. Furthermore, there are two bugs listed from just three days ago, one of which mentions Youtube videos playing without audio, the other one stating, "Lightspark failing with some flash videos."
So, while it might be a good candidate for a replacement in the future, at this present point in time, I would think not.
DISCLAIMER: I have not used Lightspark; my thoughts spring only from what I read on that website's main page and on the software's bug list.
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