I use a Fetch Mighty, and it has been working great for the last 2? years. When I first got it, it had an issue with no picture, when first turned on, but maybe due to a firmware update, it hasn't done that for ages.
You do need an internet connection, for it to work. And you have to register it online (cost $1.00 from memory) in case you do decide to watch a movie from one of the pay services. It also uses the net connection for programme data, which it also uses for programmed recordings, and for catchup TV.
We have a gen 2, which has three tuners. This means you can record two channels from two channel groups (4 recordings) and watch from a third channel group. The Gen 3 has four tuners, so it can record two channels from each of three channel groups (6 recordings), and watch a fourth channel group.
When you tag a series for recording, the Fetch remembers the tags, and adjusts recording times according to the listing in the EPG. It may be a few mins out here and there, but it rarely misses the start and finish of programs. You can add time to the start and end of the programmed time to overcome any of this if you want, but I've only ever had to do that for a couple of channels that have a movie after a show that notoriously runs overtime. And it's no worse, and probably better than a lot of PVRs that use channel transmitted data for the EPG.
The Gen 2 has a 1TB HDD, and I've had well over 100 recordings, with mostly movies and a few full series listed as a group in that.
Another nice feature is you can add a number of Fetch Minis to the system, and they act like a remote access point to the Mighty. You can setup your recordings, view and delete from the Minis too.
If you look at buying one second hand, you have to make sure it isn't locked to an ISP, and that the original owner has cancelled their subscription for that box, or it will be unusable for you. I recently picked up a Mini from Gumtree for $50.00, after making sure that it was unlocked and subs cancelled, and use it in my shed. No more running up to the house to set up recordings on the Mighty :)
One downside is that I haven't found a way to transfer recordings to external storage media, which would be nice for concert recordings.
Overall, it's probably the best system I've used, and I'm more than happy with mine.