Which NoSQL database should I use?

I am interested in trying NoSQL database but there are just so many of them these days.

I want to use it to store threaded comments (like blog comments, where users can post a nested reply to a comment) This sort of thing is surprisingly difficult with MySQL (I personally know at least 3 ways to do it in mysql)
With NoSQL this is easy since you basically store json object.

My other requeirements is that database must be easy to work with from php. So I looked at MnogoDB, CouchDB and Cassandra.

I liked many featured on MongoDB but there are several important limitations like around 2.5 Gig total database size limit on 32bit OS and I don’t want to upgrade my OS just to install a new database that I am not even sure I will like.

Another surprisingly stupid limitations on MongoDB is a global lock. You know how you hate it when you MySQL locks thw whole table just to update one row? Many people started using InnoDB storage because of a promise of row-level locking only to find out that everything else about InnoDB sucks.
Well, if you hate the table level lock, how can you justify the global lock?
Sure, MnogoDB locks are released propbaly 50 times faster than in MySQL because the write are not really writing to disk (which is another can of warms, but I can live with that idea)

And lastly there is a 4MB per document limit in MongoDB and they promissed to incsease in to like 32MB in the future, but not sure when.

So I think MongoDB is not good enough.

That leaves me to choose between CouchDB and Cassandra. Both look great, Cassandra looks more advanced, and in CouchDB I kinda like the idea of HTTP as main communication protocol.

Have anyone actually tried more than one of these NoSQL databases and can tell from their experience which one is better?

A developer named John Paul Ashenfelter gave a talk about NoSQL at cf.Objective() this past year. It was pretty good. Here’s a link to a PDF of his slidedeck…perhaps it will help you?

PDF: http://www.cfobjective.com/cfo/assets/File/pdfs/JohnPaulAshenfelterNoSQL.pdf