I have never owned a Dremel before, I'm planning to buy a Dremel VersaTip and one of the multi-purpose tools (300/400 Digital series).
First of all, are these Dremel tools really useful?
I would have to say 'Yes', you will probably find yourself reaching for it more often, once you get comfortable using it. In fact, some years ago I had a complete refit to do on a 20ft boat. When I say 'Complete', I mean the whole interior was stripped out down to the bare fibreglass hull. The Dremel and a couple of carbide cutters handled it well.
Do the bits crack and break really easily?
The cutting wheels can crack and break if side force is exerted upon them, which is not how they are designed to be used. Always use eye protection..
Is the VersaTip actually good for soldering/welding?
Sorry, haven't used it so am not able to comment.
The tool package itself is not very expensive but replacement parts look overpriced. Is it all worth it?
The price of replacement bits are how they recoup the profits from offering a reasonably priced kit with minimal bits. 'Worth it' is subjective and depends on what the user intends to use the tool for. Using the bits on inappropriate materials will quickly dull/destroy them and result in more frequent replacement - more expenditure.
Which Dremel multi-purpose cutter should I get, 300 or 400 digital? Should I get one of the large packages with a lot of bits, or stick with the smaller ones?
You should weigh up your options - will you use every bit in the larger kit, or will you benefit more from hand selecting bits as each project requires.
What accessories (e.g. the tiny work bench) should I get?
If you currently, or intend to, make your own PCB's, a drill press attachment is pretty much invaluable. Expanding upon the boat project mentioned above, the little router table attachment came in very handy for making an instrument panel and flush mounting gauges, switches etc....could you see a benefit in using this for project boxes?
Please do not hesitate to call any of the above mentioned products crap if that is the fact.
Thanks in advance,