I'll try to answer this (and I'll e-mail, too, in case you 
don't check this forum that often ^_-) ~

Firstly, you didn't specify ANIME cels or Western cels. I think 
they're very different markets, with different monetary 
expenditures from different collector audiences, probably 
partly due to the different targeted audiences of each medium 
(Anime vs. Western cartoons). Therefore, being a strict Anime 
cel collector, what I say here is biased for Anime cels. 

*I HIGHLY RECOMMEND POSTING TO THE CELMAIL NEWSLETTER TO GET 
VIEWS FOR WESTERN CEL COLLECTING, AS WELL.* SOME GOOD GENERAL 
ARTICLES (mostly for Western cels, but much for Anime, too): 
http://www.animationartgallery.com/animationarticles.html

Cel collecting is not very popular compared to other merchandise 
for the same series also for various reasons - money, time, 
rarity, etc. 

I personally would rather not get into "what's hot" and that 
sort of thing - cel collecting is a specific and difficult 
hobby as it is, once we get into "marketing" (there was 
discussion about this about the negative influence of EBay, 
for instance) the prices on various cels will change drastically - 
largely because of inexperienced and/or ignorant newbie hobbyists. 
(This is just my take, of course.) 

I don't like thinking about specific cels as "THE NEXT BIG 
THING" because it isn't about fashion - granted popularity 
of specific cels will depend on popularity of a new show, but 
most people collect for : 

1)Characters (favorites, etc.), 

2)Good image (whether in "quality" - which is subjective but 
there is an actual set of "guidelines" for Anime cels: 
http://www.kimonocels.com/Animation/pricing.html

3)BECAUSE YOU LIKE THE PICTURE (yes, redundant)- the "guidelines" 
above are still strictly subjective, some people like action 
shots, for instance.

Cel collectors generally agree that to collect for "investment" 
purposes is a bad idea. Cels were not meant to be Fine Art 
collections. Cel collecting (at least for Anime) - is a HIGHLY 
PERSONAL THING. 

I see EBay as evil, I won't get into it here unless you ask 
specifically - the latest thread on the Forum with opinions 
is the "Gundam WIng cels on Ebay: Okay, I know this came up before 
but.." thread (do a search). 

Also - there can be a big pricing differential between Western 
cels and Anime cels. At least that I'm aware of. One can easily 
get some Anime cels for less than $10 USD. The most expensive 
Anime cel I've heard of (it's mythical!) is $10,000. The most 
expensive cels I've seen tend to go for a few thousand for 
ultra-rare and ultra-desireable characters. 

Addicting?.... ACK!! We tried to figure out why it's addicting...
I started a thread called "WOW. I wish my apartment was this 
clean. ^_^ hee!" which was a survey that tried to answer this. 
I think it has something to do with the collector's mentality - 
a measure of possessiveness (it's MINE! I *OWN* that split-second 
of a frame!!).

^_^