upsides and downfalls?

need4speed

I'm not an owner quite yet. Waiting for the 40th anniversary model in 2010.
I've test driven this car a few times extensively. But what I can tell you is that your question is extremely broad as far as upsides and downsides. We don't know what your preferences are?
Just remember a few facts:
The Z is a 2 seater with little space.
Its a loud car in engine noise and what some call harsh sounding. (I like)
Its not designed well for fat guys. (Tho many are attracted to the struggle of owning one) lol

As far as modding goes:
Nissan is a expensive car to mod that proves very mediocre results unless you aspirate this vehicle. An average full mod at the end can be 18k for average 150hp gains.
Plenty of other cars mod simply, cheaper n easier.
If u a serious modder. Look at the genesis coupes b4 buyin this.
You might be impressed?
The flipside to mediocrity in the mod department is the incredible stock performance.
This is a poor mans porshe. Extremely fun and reliable.
With minor suspension tweaking and bolt on parts. U can have a needed boost in low end torque and will make for a most enjoyable tossable whip.

I hope my input helped at least a lil bit.... peace.