I picked up MgT HG a couple of weeks ago, so I might be able to help.
Does MGP High Guard add much to the game?
In general, yes.
The advanced character rules are a bit wonky in places. For example, the Imperial Navy produces doctors more by luck than design and intelligence operatives are a minimum of 34 years old, even for first time field operatives. I think the minimum terms prerequisites are probably best replaced with minimum rank prerequisites (so a character who has gotten lucky and been promoted quickly doesn't have to wait three terms before he can enter Command).
I have not read much of the new starship combat rules (mostly on handling the hundreds of weapons of a capital ship and vector-based movement) and adventuring while still in the navy.
I understand it is all about the ships in space but is the new ship design rules such that a non-gearhead could use them?
]
This depends on how easy you find the starship design rules in the core rules. The Spacecraft Options chapter mostly just adds more options, but isn't too complicated. The chapter on small craft is really just a smaller-scale version of the regular starship design system. The capital ship designs system is mre gearheaded than the core rules and unfortunately applies to ships as small as 2,001 tons.
Does MGP High Guard make you want to design new ships?
I got it because I wanted to design small craft and ships larger than 2,000 tons
I am not a big fan of the CT High Guard as the design sequence made my head hurt so I only used the CT LBB design rules.
You can probably use the small craft design rules without needing aspirin. The additional starship options are mostly easy to use, although you might find the munitions types for missiles, torpedoes and railguns to be overkill (you can probably just stick with the standard warhead for each type). The capital ship rules ... well, I think they are more complicated than they need to be (with ships being divided into sections and making up hit locations tables specific for each design). It reinforces my small ship universe mindset and makes me prefer to just extend the hull and drives tables in the core rules up to 6,000 tons.