I've always put a ROM on my android phones, generally because they ran crap and battery life sucked, just got my S5 today, so far so good, wondering if I could just leave it alone as is?

I would leave it alone to start with at least. See how you get on with battery life and whether you find the stock features useful.

I've had mine quite a few months now and have it with the stock Three software on it although I have put Nova Launcher on it as it does seem to make it very slightly snapper (even though it was not slow to start with, but also love the extra row of icons and also transitions between screens.

Lollipop for the S5 is rolling out in Europe as we speak, it should reach handsets in the UK like yours by the end of Jan its expected. This also apparently extends battery life as Samsung has took advantage of the new code in Lollipop which extends battery life by as much as 30% (some cases even more).

Check for the latest updates as they always bring new enhancements. I have a Note 4 and admittedly battery life is stellar so I can't wait to see what difference Lollipop will be

In my view the stock S5 rom feels way better than stock rom's in older samsung phones, I think samsung recognised the bloatyness was bad and have been working on improving their default rom's

This morning I added a CWM recovery to my S5 (my version of the S5 only have a half baked recovery tho as is almost no dev support for it), I backed up the stock rom and put the backup on my pc, and after its charged (since I not charged it yet since brought it) I am going to root it.

I will then use it on the stock rom rooted and see if I am happy with that.

Most custom touchwiz roms are usually just removal of apps considered bloat, and some tweaking. They sometimes also come with custom kernels. In my experience its the custom kernels that provide the most benefits, the debloating can be done with apps like titanium backup to uninstall system apps.

Of course if you prefer AOSP based roms, then thats an enitrely different thing since the only way to get AOSP on a samsung phone is to replace your stock rom.

Also worth pointing out these new phones have a KNOX counter, if you root or replace the rom then that gets changed to 0x1 and will void your warranty.