I bought a 350z in December, and I'm really enjoying it. The gearbox is fine imo, coming from a e9x 3 series (i found my bmw box mushy in comparison) you have to be a little more precise when it come to fast shifting, apart from that i've not had any mechanical issues (I preemptively changed the gearbox mounts to polybush out of want that's it)
With regards to weight i wouldn't say that it feels heavy just more solid and if you're really hammering it you'll probably feel a bit of body roll more than weight itself, its nothing that cant be fixed with sway bars or something.
Power wise I'm still really enjoying and and don't feel it needs more, you can oversteer any time you like but in the dry you have to make it it happen (unless you're running horrendous tyres). In the wet with TC off you defiantly have to be conscious of your right foot as I've had to catch it a couple of time but its more funny than scary and it's nice to be kept on your toes.
Also it's worth mentioning that I do all my work myself and part prices aren't too bad and if something is a bit pricier there are plenty of people breaking them or used bits for sale on ebay. Its worth looking at part prices for porsche and coming form a bmw I can say part prices are a lot were worse and mine loved to pop fuses and whine about abs.
It's worth looking about for a 350 as i got lucky and mine doesn't burn oil (as proper ragged ones are bad for). Also if you didn't know these chug fuel and tyres are expensive ( at least for my 21 yr old self!) I still think it's totally worth owning and one, if you do decide to get one you'll love it!