Hi,
I happen to use them, and I think they're OK. My DD is very picky and it took me ages to find a nipple that she liked. I almost had to buy all the brands in the market in order to find one that she would like.
True about taking longer than usual to heat, but what I do is that I have a bottle heater next to my bed and I leave the bottle heating at the lowest temperature (DD likes her bottles warm, not hot). Also, an advantage of the plastic being so thick is that it keeps the milk warm for longer.
Also true about the spilling, but what I do is that since I take the bottle out of the bottle heater that has water, I have to wipe them anyway, so I don't have any extra spills.
They have a lot of parts due to the air valve, but I put up with that because thanks to the air valve DD swallows a very little amount of air when eating and therefore, I don't need to burp her. Also, by washing part by part I feel more confident that the bottles are 100% clean - without soap/milk residues.
About the space thing.... well, I never thought of that.... I have a microwave steriliser that will only hold 3 bottles at the time, so I have to sterilise the bottles in various batches anyway ..
Good luck!