Yes you can download Serato DJ for free and it will automatically activate once it detects that you have plugged in the DDJ-SR controller.
The F/X packs can be bought directly from the extension pack manager within the Serato Software.
Well there are already loads of effects in Serato and a free extension pack to download. If you still think you need more, then of course you can go for it.
Are they worth it? Well if you need to use them, then yes. iZotope is known in the recording industry to make the best studio quality effects out there, so yes they are extremely high quality and in that respect beat any software effect found in other DJ software.
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