What is the lifespan of an SSD drive?

SSD is so new in the market that while there are published research, it is currently all theoretical conjectures. I refer you to some here.

SSD Myths and Legends - "write endurance"

Are MLC SSDs Safe in Enterprise Apps?

Flash SSD Reliability

Anandtech's Review of the Intel-X25M

There are many others online, you can do a search (sorry, too many to list all down here) and find out more also. However, the gist of all articles is SSDs are more reliable than hard disks, and should last a good 20 years at least not counting performance degradation.

The answer to your other question of similar reliability level among SSD manufacturers is a resounding "No!". SSD manufacturers find various ways to cut costs by using cheaper NAND chips, controllers, QC process. Go for brand names like OCZ, Intel and Samsung - they are so far considered the most reliable on the SSD list.

Are SSDs comparable to HDDs for reliability and is there published research on this?

No, they're not comparable. SSDs are shock proof which itself puts them light years ahead of any platter hard disk.

And here's a statistic for the average lifespan of various data storage media:

Platter hard disks: 3-6 years

Magnetic tape: 10-20 years

Floppy disks: 1-5 years

Optical disks: 10-100 years

Static memory (such as SSDs): 50-100 years

Stone tablets: up to 10.000 years

Source: Wikipedia (for the stone tablets :) and ZDnet (for the rest).

Of course your mileage may vary, depending on the use. The one fact in favor of platter hard drives are the relatively low costs compared to SSDs, robustness and performance comes at a price. And from personal experience I can tell: The SSD beats the platter hard drive hands down, in terms of speed and reliability.

Jeff Atwood (co-creator of Stack Exchange and this very site) wrote an article stating that SSDs are NOT as reliable as the other answers here suggest. He mentions a friend who has had eight SSDs fail on him in just two years!

I do wonder if that person's failures had some other unknown cause in common - maybe a bad power supply feeding them the wrong amount of electricity or something? But it's an important data point anyway.