Undeniably, Hydras are one of the most awesome vehicles for the Imperial Guard. It's the ideal killer for AV11-AV12 vehicles and a couple of them have high enough rate of fire to convincingly stun lock an AV 13 tank (fantastic against Predators).
They are also quite easy to protect. I run a squadron of two, using the Forge World models. I just need to park a chimera in front of one (no LoS blockage for the main guns and if placed correctly secondary gun will be OK too) and I instantly get 4+ cover for the lot. They have an amazing range (72'') so I usually deploy them way back so I don't worry about meltas usually until turn 3. And they are also cheap as chips costing 75 points each. They are very often the MVPs on my games. I think every Imperial Guard army should have a couple.
My main issue with this vehicle is what secondary weapon I should give it, Heavy Flamer or Heavy Bolter? I figured out that since I will be mainly shooting AV11 - AV12 vehicles, a heavy bolter would be almost useless. So I had Heavy Flamers in my squadron just in case something got too close. Also not very keen on BS3, at least with the Heavy Flamer you know you are going to score some hits.
From the games I played I realised that usually after the second turn I was shooting at infantry most of the time with the Hydras. I never got to shoot the Heavy Flamer even once. Heavy Bolters seemed like a much nicer alternative, since even with one hit I would be doing something.
If a unit comes at me outflanking it will probably kill the Hydras if it is in range (meltas or charging). If it is not in range then the Hydras are not going to be in range for the Heavy Flamer to work well so there is not much point to it for this case. And since the main weapon is 72'' I always had a safe distance from my targets.
Of course I will need to keep my Hydras stationary to use both guns. I don't think this is a major issue since they shouldn't be moving unless they got an imminent charge incoming. Sometimes it is necessary to redirect them to more vantage shooting points but if they were deployed correctly from the beginning this shouldn't happen too often.
I posted this question in a couple of forums and people seems to agree that the Heavy Bolter is the better option. For my last few games I've been using it and it seems to do the trick. I was also thinking about giving them extra heavy stubbers since that would make them very good against infantry. Unfortunately you can give so many good stuff to your units that there are not many points left in the end for extravagances like these.
On a different note, I got the first vehicle for my Space Marine army today. I headed down to the city-center Fantasy Shop to use the cheque I won at the tournament last week. Very excited to actually get something that I earned by doing well in a competition. I know it's not much but the feeling is great. I got a Land Speeder and since there were 2,5 euros left from he purchase I also got a Vallejo Goblin Green which seems ideal for a Salamander's mid tone. The kit looks very easy to put together and I will do it on the first chance I get. I have to finish my models for tomorrow's Warbands league game and I have to make sure everything is WYSIWYG so not enough time for my pet project tonight.