I have one and I like it. I did take advantage of the trade-in offer at Gamestop, trading in my DS Lite. I'm not sure I'd pay for it outright, though. It is a very good improvement over the DS Lite in a variety of ways (with the obvious exception). In the end I ended up also buying a used GBA SP (brighter screen) from ebay and actually I'm really glad I did. Not only can I play GBA games, but GB and GBC, plus hook it up to our Gamecube and also my son and I have played some two-player GBA games, none of which you can do with a DS Lite. So in the end I'm really happy with this combination, but if you don't already have a GBA SP, you've got to factor that into the equation.
PictoBits is really good. My other DSiWare downloads have only been fair, but PictoBits gives hope. It is really nice having PictoBits on there all the time and not having to carry a cartridge. I wish they'd offer full DS games as downloads as well. There's absolutely no reason why they couldn't. There's a number of games I'd be willing to buy as a download.
Better, of course, would be some system for being able to buy games on cartridge and then load them onto the DSi. This, I believe, could be done via hybrid cartridge that when loaded on a DSi allowed you to copy it to internal memory, and then it would set a kill switch, of sorts, disabling the cartridge from use on a DSi or DS Lite. You could then re-enable the cartridge by "unloading" from your DSi back onto the cartridge, thus still allowing for reselling or lending games, and avoiding piracy (sure, somebody will work around it and pirate the games, but that's happening like wildfire anyway even without this feature - this scheme would allow for people who don't want to pirate to be able to legally install onto their DSi, which is uber nice, and probably would reduce piracy in that it would give people on the fence less reason to go the illegal route, which inevitably, regardless of initial intentions, leads to outright piracy - I know that from experience!) This would be a super wonderful thing, but I doubt they are ever going to do it.