Cinelli are classic bikes. a google search will reveal to you they are one of the most storied brands in cycling. they may not have the same 'race pedigree' as colnagos, pinarellos, bianchis or williers or other italian marques but they do have a rich history. They made a great name making quill stems/bars/seatposts/steel frames back in the 50s-60s.. in fact some of their steel frames are still very well regarded. Cinelli don't actually make very many frames- they have always been a small company and their focus is on making components, not frames, which may explain why they are rare. in 5 years of riding I can count just 5 cinelli road bikes I've seen on the road... and I worked at a shop that sold cinellis.
Cinellis have a huge reputation in the track/fixie/hipster bike world and they are 'cool' that way. Check out the bootleg mystic rats- very cool bike.They've always wanted to do their own thing rather than fight with the big boys in the all out competitive cycling arena, which explains alot of their quirky and unique paint schemes. they don't try to out advertise, they don't sponsor big cycling teams, and my impression is that their brand culture has always been abit 'underground'. their distribution has not always been fantastic and sales have suffered somewhat because of this. If you like them, and the brand culutre they promote, more power to you. Cinelli makes unique bikes whcih are nice and certainly won't hold you back. They CAN be found if you're willing to look hard, just more difficult to find than brands like specialized and cannondale.
Frame warranty is mostly a standard two years across the industry ( for most mainstream brands- I know boutique builders have sometimes 5, 10 or even lifetime warranties for steel/Ti Frames, but you also pay lot more.) Giant is the worlds largest manufacturer of bicycles and actually make bikes for scott, colnago, specialized, and many other companies out there. if they need to warranty a frame, it's a tiny drop in the ocean for them compared to a smaller maker like cinelli. Know what you are buying. Google is your friend. Giant make perfectly good bikes, albiet without the lust factor of those 'supposedly made in italy' bikes. personally, I wouldn't choose a bike based on warranty. I wouldn't choose a carbon bike as a forever bike either.
Ti versus steel? what you're paying for is the added durability of Ti, and the difficulty of working with it. Steel WILL RUST. even if it is 'stainless'. of course there are better grades of steel and you can protect it with paint and what not but steel will die, eventually, whether it takes 10, 50, or 100 years. Ti, if done properly, can be made to be virtually indestructable ( other than the sudden and violent application of force) . it's much more corrosion resistant than steel. ( much, MUCH more) but is also more difficult to weld and machine. and good quality Ti is expensive. Steel is much easier to repair if needed, though. Ti is not the stiffest material for a race bike if you want that. different materials will have different properties.
As for willier, they make good bikes, also with a long storied name in cycling. Aesthetics are relative though, and I don't like how they look. once again, google is your friend. But really, 99% of large bike companies make perfectly good bikes nowadays. You'd really be splitting hairs. Basso is one of the rare few bikes actually still made in europe and not taiwan/china but I don't actually think that matters anyway. their finishing may be better but the Asians- particularly big factories like giant and merida- are the experts at making bikes with carbon. narrow down your search with the appropriate geometry, and then after that choose something you lust after and test ride it. it's really that simple. buying a bike is not rocket science.