Pros

• Inexpensive storage
  SD cards are really cheap nowadays and they are easy to find. A 1gb sandisk that costs about 15$ holds all the HVSC archive and all my games and still have 500mb free space. 

• Really fast loading times
  Faster than 1581, 64hdd and ide64. Actually the only thing it cannot beat must be a cartridge =)

• Cheap, cheap, cheap
  Considering the other options, this is the most cost effective modern storage solution for c64. Shipping is also cheap because of its:

• Small size
  It fits in a standart C= cartridge case. It just needs to be slightly modified to make an sd card slot. It is pretty light so you dont have to put something underneath the cartridge for the fear of breaking the edge connector.

Cons

• Closed source and slow moving firmware
  Documentation is scarce. The firmware is being developed by a single guy, Oliver Achten. While it is a big feat of programming it slows down the process. 

• Mostly readonly access
  The only tool that can write to sdcard is a floppy reader. It reads the inserted disk and converts it to a d64 format file. There are no other software i know of  that can write on sdcard.

• No 1541 emulation
  And this means no fastloaders, no multipart demos. This is really a big downside. 

Last Words 

MMC64 from Individual Computers surely is a big step forward on the way to eliminating long loading times from increasingly hard to find media. Even though the hardware is working really well, I believe the available firmware is far from complete and  an opensource model should be considered for firmware development. All in all MMC64 was a great buy for me.

If you also happen to have a Retro Replay cartridge "MMC64+RR disk image mounter" allows you to mount d64 images from your SD card. Many games are known to work with this setup:

MMC64+RR Disk Image Mounter @ SymbolEngine
MMC64+RR Disk Image Mounter @ CSDB

If don't happen to have one fear not! For there is "Sasq's alternative BIOS" that can  load some ide64 fixed games.It's not as compatible as the former but what can you say?

Sasq's alternative BIOS for MMC64 @ SymbolEngine
Sasq's alternative BIOS for MMC64 @ NightMode

Download it here: Ascraeus Font Editor V1.2

I feel obliged to spit out an example of this powerful stuff. And when you see the power of scripting combined with assembler on a single source file, I believe you will never look back. This code below is the mandelbrot example that is in the zip file. 
It calculates the mandelbrot set and fills character memory accordingly

.pc = $0400
.function toRe(i) .return -2+2.7*[i-floor(i/40)*40]/39
.function toIm(i) .return -1.6+3.2*floor(i/40)/24
.function mandelbroot(re,im) {
.var zr = 0
.var zi = 0
.var iter=0
.for(;[zr*zr+zi*zi]<4 && iter<20;iter++) {
.var newZr = zr*zr-zi*zi + re
.var newZi = 2*zr*zi + im
.eval zr = newZr
.eval zi = newZi
}
.var colors = List().add($20,$0f,$20,$08)
.return colors.get(iter&3)
}
.fill 25*40,mandelbroot(toRe(i),toIm(i)) 

Edit: Kick Assembler now support user defines pseudo commands (Yeeeeaay! 16 bit operations with one opcode!) and DTV specific opcodes.

Impressive isn't it? Here are the links if I convinced you: 

• KickAssembler 2.25 Homepage
• KickAssembler 2.25 @ Symbol Engine

• Here is the original: Commodore 1541 Floppy Drive ROM dissasembly
• Here is my local mirror: Commodore 1541 Floppy Drive ROM dissasembly @ Symbol Engine

Relaunch64 IDE
This IDE is strictly for C64 assembly crossdevelopment and has very nice features. It has syntax highlighting and can easily be configured to use a number of CrossAssemblers, Crunchers and Emulators. The editor has some weird bugs, but if you dont want to configure giant editors like EMACS to do what you want, then this is what you want.

Exomizer Cross Cruncher
You know those loading times on C= machines can drive anyone mad. A cruncher compresses your executable code and adds a decompressor at the beggining of the file. But crunching involves heavy calculations that is better done on a fast machine if you are fond of your sanity. Exomizer 2 is a cross-cruncher. It is full 32-bit code that works on intel machines, and it can crunch executable files for VIC20, C64, C16/plus4, C128, Atari 400/800 XL/XE, Apple ][+ and //e. Nice eh?

VICE Emulator
And of course you wouldn't want to transfer binaries from PC to your C= too often. This is a multiplatform emulator that is *almost* as good as the real box. Still you would want to check your final binaries on a real C=. Just in case…
The current version emulates the C64, the C128, the VIC20, all the PET models (except the SuperPET 9000, which is out of line anyway), the PLUS4 and the CBM-II (aka C610).

So don't frikking sit there, Go 8-bit!