Can anyone offer any advice on what features to look for (and what to avoid) in a PC based oscilloscope.
My platform is Windows 7 64bit so driver availability may be an issue.
I've downloaded the software for a Hantek DSO-2090 which installs and runs OK, but obviously the USB drivers don't install without the hardware.
The FFT feature certainly looks useful.
Does more expensive mean better software, or is it a case of the same software being packaged with more expensive hardware?
You need to specify what type/speed, number of channels of signals you want to look at before asking what to buy.
PICOTECH.COM make some pretty good devices in all range of specs and prices. Even their very cheapest DAQ units offer quite good oscilloscope features.
In order to give any recommendation, you should probably include features and capabilities important to you. Otherwise, recommendations are completely random.
In general, my advice on PC-based scopes is to avoid them. Unless you are trying to setup some type of automated measurement environment, they tend to offer few advantages to a stand-alone unit. No knobs or dedicated controls means you have to have room on your desk for a mouse (and keyboard.) "Glancing" at the screen becomes troublesome when the scope application isn't running or forefront. PC-based scope software typically doesn't offer any more measurements than low-cost stand alones. The cheapest PC-based scopes stream sampled data over USB (no "local" sample storage) so your sample rate becomes limited (and in some cases, unpredictable.) Generally these don't have any kind of pre-amplifier front-end so their signal sensitivity is limited (along with their impedance.)
Look at a stand alone unit from Rigol or even SeeedStudio's DSO Quad. Full functional scopes at a low price and good build quality. (Better build quality than many PC-based scopes.)
I bought a PropScope USB Oscilloscope, and it's working OK for me.
It's open source too, based on Propeller microcontroller.
Anyway, I find hard to decide what to buy. Maybe it wasn't the best choice.
I use this one, www.DPScope.com, handy little device for checking out occasional signal.
And lets face it, most of your arduino problems will be software related.
$70 kit, $10 shipping (in US, $20 international), probes included.
Sure you have to look at your (laptop for me) screen. Not so worried about physical knobs to twiddle.
Virtual Analyser is also useful for sound just using your PC sound card - I used it with a Sony microphone to look at/analyze sound from a tube amp I made, helped me determine that a lot of what I wanted to get rid of was power supply 60/120/180 from the rectifier tube, changing the heater voltage and watching the output get cleaner was very helpful.
I have not tried feeding digital signals, I use the dpscope for that.
The alternative is a low cost 'real' 'scope like the ones from Owon.
I have one of these (20MHz) that cost around 180GBP (250 USD). It has USB out too, and a fairly crude PC scope app, that I can use when I want to capture a waveform onto my computer.
I have a messy workbench so I don't have to find room for my laptop and wait for it to boot 
I use Bitscope -> http://bitscope.com/, currently considering upgrading to Agilent 2702 -> http://www.home.agilent.com/agilent/product.jspx?nid=-34492.774929.00&lc=eng&cc=US PC-based scopes are good for documenting; for real development nothing replaces bench scope. I'm currently working on digital camera attachment for my Tektronix 7904 (and also 7104 and 7934) so that I can take pictures of single-shot waveforms.
At the moment I need something for debugging interference issues with various analogue and digital sensors in a home SCADA system.
In the near future I can see a need for debugging serial comms so being able to store lots of data is a big plus.
After more research, I've found that the low-end PC scopes suffer from small buffer memories and USB bandwidth limitations and the high-end PC scopes with lots of buffer memory are more expensive than a real scope, which leaves me looking at a non-PC scope like the RIGOL DS1052E.