I haven't taken the plunge and bought the HF coils just yet but I have had the 13" coil for a little over five years now. I have an adapter which moves the mounting point to roughly the center of the coil which helps with the balance but it is still a little bit nose heavy.
Having said that, and realizing that I (and Norvic) are probably part of a very small group that feels this way, I love the coil. The added ground covering ability comes in handy when searching large parcels of land and the separation, while its ability to separate non-ferrous targets from ferrous is not on par with the smaller 9" coil, is still pretty phenomenal for a coil of it's size. Not to mention what Norvic previously posted regarding the 13" coil's ability pinpoint; "when you stand the coil up using the tip or tail with fairly shallow coins and trash you can pinpoint the coin accurately and differentiate it from trash because the ID works well even with just the tip or tail, more so than the other LF coils."
While I'm not one to post depth differences between coils due to the many variables that have to be factored into the results, I will say that when compared to the standard 9" coil in dirt that ranges from 0 - 4 or maybe 5 bars on the mineralization scale the, 13" coil wins hands down on several targets, especially when utilising the detectors lower frequencies. The following settings are for fairly clean areas but can be easily modified to handle fairly small amounts of trash: the 13" coil loves 4 khz, -1 silencer, spot on GB if not a point or two lower for even more depth and 0 reactivity if the amount of trash targets will allow you to get away with it. I'm sure I'm forgetting a few things. If so I will revise this post as I remember them.
HH,
Chris