define "good"? The Canon XF305 specs indicate it weighs 3.5 pounds. That would be with the stock battery, no mounted external mics or wireless battery-powered portable base stations or video light or monitor... I've used a couple of different models with a Sony HVR-Z1/HDR-FX1/HDR-FX1000 (around 4.6 pounds), Panasonic AG-HVX200 (around 5.7 pounds with battery), Sony HDR-HC1/HVR-A1U (about 2 pounds) and Canon HF S100 (about a pound).
The different shoulder mounts all work about the same. And after a few minutes your arms will be tired whether the shoulder-mount has you using one hand or two. Even the ones with the counterweight on your back and those which have a pole-mount to a belt... I don't know what you plan to be shooting, but if it is anything that can take a while, I suggest that you save your money for something more along the lines of a vest-type counter-weighted arm like those from Steadicam, Glidecam or Varizoom... or even one of the overhead suspension systems from Easyrig. Yes, they are LOTS more $, but you aren't depending on your arm strength. And they have the advantage that walking or running with them can all you to capture very stable video (the shoulder-mount systems won't do that).
On the other hand, if you are capturing relatively short sequences and you don't have to stand around being ready much, then (as previously stated), the shoulder-mount add-ons all work about the same. Of the ones I used, I like the SpiderBrace 2 - and my home-made system.
Another issue (other than arms getting tired) is a pro-grade shoulder mount cameras' eyepiece is where the LCD panel flips out on all of the camcorders listed above (including your XF305). The SpiderBrace2 (and my home-made system) allows comfortable use of the eyepiece at the rear of the camcorder (the LCD panel can also be used).