I bought my own (very expensive) Gore-Tex bivvy bag when first joined the Army, but it didn't see much hardcore use as we were issued them soon after. I still have it, and it still works well, athough it has an L shape zip and zipping yourself up completely isn't everyone's cup of tea.
The Army issue ones are very good - no zip, but they have a drawstring which allows you to draw the top up tight, leaving a small hole to breathe through, so the bag doesn't get moist with condensation.
In my experience, a bivvy bag alone won't keep you dry in prolonged heavy rain. Gore-Tex does let water through eventually, through zips, seams and small holes, and once it's not brand-new or recently re-proofed, throught the fabric itself, as it wears and the membrane starts breaking down.
To get a comfortable nights kip in a bivvy bag, you really need to be under shelter - for example a PU coated nylon poncho bungied by its corners to four trees, or in a bus shelter or cave. You also need a camping mat, although a shoulder-to-hip length one does the job.
I'd choose a lightweight hike tent almost every time these days. My North Face Tadpole weigs 1.7 Kg all in, and is big enough to change and sit up in, and you can cook in the bell end. It's not that stealthy though, and if you're camping in places where you'd rather nooone knew you were there, a bivvy would have it's advantages.
I did borrow a double-hoop Goretex job once and slept in it for one night. They were touted as the dream solution for military field use, but I found I prefered the Army issue one, being far less of a faff, and less prone to condensation provided your mouth wasn't inside it.
Slugs? No problem. Just don't eat them, they can come with some really nasty parasites.
