I bought one of these from an amazon lightning deal about 3 years ago and I've been lucky enough to have very few problems with it.
For streaming TV content over the wifi/LAN in my place, it works great. With the browser plugin or PC client, the video quality is great and turns your device into a second HDTV (sort of).
Streaming quality on the go is really heavily dependent on your internet connections upload capabilities. If your home Internet supports 1.5+ MB upstream, you should be fine viewing away from home. Anything less than that and you will see a definite streaming quality degradation. However, the player software (desktop and browser) have adaptive streaming that seems to work pretty well.
I can't attest to problems with the capacitors, but I've been lucky enough that my device has worked this long without any real issue.
And yes, sling does charge for support incidents (which sucks). And they charge $30 for each mobile app platform. The iphone/ipad one is $30, the windows mobile one is $30, the android phone one is $30, the android tablet one is $30, etc... which sucks. And they don't support the desktop clients for PC or MAC anymore (which sucks).
The worst problem I've had with the device? The desktop player and browser plugin have to phone home to the Sling "mothership" and authenticate your account before you can connect. When Sling's servers go down (as they did for about a week in August 2010), your slingbox becomes a fancy paper weight.
"A note if you have a Scientific Atlanta 8300HD box connected to your TV via HDMI, plugging component cables from your cable box to the Slingbox will result in audio only and a "no video detected message on your laptop/mobile device. Seems that particular cable box is designed to lock out simultaneous use of both connections. Not sure if that's the case with all HD cable boxes as this is the only model I've ever had."
A note about this: This is not a hardware limitation of the SA-8300HD box (and most other cable boxes, etc.), it's an HDCP restriction placed on the cable box by your television content provider. Generally speaking, if you have your cable box connected via HDMI and the device completes its HDCP "handshake" (ie. it's plugged into something turned on, like a Logitech Revue or an A/V Receiver), component output is disabled. This is a software control pushed down, and within the last few weeks my cable provider or (junky) cable box has lifted this restriction.
On a final note, the HDMI support on the SA-8300 is *awful* and if you're connecting to your TV via HDMI with one of these boxes, I suggest you swap out for a new one.