84+:
Amount of games: A+
Amount of quality games: B+ (Thanks to low screen res and crappy RAM)
Programmability: A- (lacks C and BASIC is slow ish)
Audience: A+ (Most popular calc series around and can run 83+ games)

89T:
Amount of games: A-
Amount of quality games: A+
Programmability: A- (No Axe Parser, but has C. BASIC isn't as easy as on the 84+ but is less limited)
Audience: F- (Almost nobody use 68K calcs anymore in the community it seems, like the 81, 82, 85 and 86)

Nspire:
Amount of games: C- (B+ if you own two TI-Nspires, one Ndless-compatible and the one with a Lua OS. Otherwise very few games are available)
Amount of quality games: A- (Even more quality than 68K, like nDoom and gbc4nspire, but still in much lower amount)
Programmability: E+ (Lua is all cool, but cannot achieve games like nDoom or GBC emulators. BASIC cannot do decent games for the most part and ASM/C is not available on recent OSes. TI constantly tries to block ASM/C on it)
Audience: D- (declining at an alarming rate since OS 3.0 release that blocked Ndless. Notice how Omni activity dropped a lot compared to September 2010)

Prizm:
Amount of games: F- (only 5 or 6 games, although 9860G BASIC games sometimes work on it, although much slower)
Amount of quality games: D- (Very few of them, although Obliterate is excellent and PrizmCity looks promising)
Programmability: B- (Basic graphics are unbearably slow and Axe is unavailable, but there's more freedom for ASM and C and eventually we might have more languages such as Lua, Java and some sort of Axe hybrid)
Audience: D- Extremely small audience, but slowly growing and might grow even more now that the calc is cheaper than most TI calcs.

9860G:
Amount of games: B- Not a lot of games
Amount of quality games: B+ Quite a good selection of quality add-ins, although most are unfinished
Programmability: BASIC, ASM and C. I think Lua is being developed too. Basic graphing isn't that fast but is much faster than on the Prizm. OS 2.0 has string support like on TI calcs.
Audience: E- Very few  people using them it seems and not alot of people interested in programming for them anymore, except maybe BASIC. However, most BASIC games are Prizm-compatible, which increases their audience slightly.

That's my general view on it, although some people might disagree/agree. I think more people should post their views in overall.

I say: Spend a bit for a PRIZM or other CASIO Device

Flyingfisch: Of course you say that they don't seem like they are that good atm...
Why should I get one?

Actually, get an fx-9750GII and hack it.

Reason:

Price: >$50

They don't crash..... lol

Pros:
actual math is speedier than TI.
They can up/downgrade without a problem unless you live in france.
I have not heard of one casio calc having an unwanted RAM, ROM, or memory clear.
I have not heard of a case where a casio calc crashed.

Cons:
Drawing speed is slower
Not as much activity (although I am trying to change that)
They are not as widely supported in schools, particularly American schools. (however, many of the functions are almost Identical to TI's, so it is not too difficult to transition.)

Oh, yes.... Thats another great option (if you have enough $$$)

Well, thats the first I heard of a casio calc crashing...