I'm just curious to see how popular the game, how smooth the rules are, learning curve, and balance of the armies.

I understand that it's GW, but it was through specialist games, so who knows.

Are the nid ships any good? I've been looking at those lately.

I was wondering this too actually, as it was originally released when I quit 40k around 2000, I believe? I never got to play it. But if it's any good i'd like to incorporate it into a narrative campaign: A game of BFG, then a Kill Team game to represent boarding parties skirmishes, then a normal game of 40k.

It was an ok game, though its balancing left a lot to be desired.
General feeling seemed to be Chaos > Imps due to better designed ships.
Eldar being a pirate fleet was meant to start with 50% of the points of Chaos / Imps ones, what in campaigns was fine as you built up new stuff along the way, but made 1 off games rather strange, Imps vs Eldar owned due to a lot more laser weapons on imp ships giving eldar saves, while chaos ones were more firing solid lumps at ships and then eldar didn't.
Orks were weird as always and later on the introduction of Tyranids was interesting.
I think it kinda sunk though like Man'o'War (what actually was a great game with a lot of balance, apart from darkelf and nurgle ships ruining it).

I still play on rare occasions, and overall the game is pretty fun.

Balancing can be an issue, but I've found that to mainly be if you try and play all of the fleets the same way. For example, the tactics for imperial fleets tends to be geared more towards several ships working in tandem. Chaos, however, has an easier time with their ships going it alone or working in pairs (two slaughter class cruisers are more than capable of taking out one or two cruisers in a single round; three can usually silence a battleship.)

In regards to Tyranid fleets, they are very focused on synapse and having a variety of options for their ships. Unless I'm mistaken, a hive ship is required and they can be made extremely tough on their own.

Battlefleet Gothic was an awesome game; I still have my Grey Knight/Inquisition fleet proudly displayed, painted up on a shelf. Each fleet had a different design behind it, and honestly, I thought they worked really well, and most of them suited their fluff appropriately.

Imperial/Chaos (since technically they were all Imperial tech) were built like old Earth Navy... most of the guns were on the sides, and everything came down to monstrous broadsides. Chaos ships, being older and pre-heresy, were faster and had access to more advanced tech than the Imperials lost, so they had better carriers and had access to more Lance weaponry, which were direct fires that could cut through armor. Imperials, to compensate as they lost that tech, went in tried and true fashion; they bolted a shit ton more armor on and made huge, slow, ponderous ships that needed armies to beat upon. Then they added tons of torpedos that could be launched through tubes in all that armor plating. They also got Ordinance weapons, making them like IG in space.

Space Marines, to show the Codex at work, only got Assault Cruisers, which were actually less durable than Imperial ships, in terms of wounds, but clearly were meant to survive any impact to get their payloads (marines) in play. While they were still good in a fight, they were clearly built to NOT be as good as real ships of the line. Their strengths were in boarding actions.

Eldar were very fun, and their strength was actually in some amazing escorts (smallest class of ship). Their battleships and cruisers were gorgeous, but they were meant to be an Eldar Raiding fleet, because the Craftworlds don't go to war. They used Solar Sails and could do a MSM just like their jetbikes, and had some amazing firepower, but they were utter glass cannons, if you could catch them. Later on they added a Craftworld Eldar fleet which had great cruisers and shitty escorts, but same basic principles.

Orks were... orky. Guns firing random amounts of firepower, ships dedicating to ramming... Honestly I'm not an ork person, I didn't pay much attention.

Tyranid ships were utterly customizable, in their role as living organisms. Another fleet VERY good at getting into boarding actions.

Necron ships were fucking amazing. The technology that Necrons had made them insane. They were the fastest ships in the game, with the best armor and defenses, and their weapons were disgustingly good, if shorter ranged. But their tech was precious, and if you lost half your fleet, you'd phase out, and took huge penalties for leaving a ship behind.

Tau were... different. In a game with massive broadsides, Tau were the ones that were designed specifically for the Greater Good. Every ship almost NEEDED other ships to support them. They were built for massed fire, and mostly front arcs. An entire fleet closing in and picking people off. FW also made their own Tau fleet, designed very much with the swept-wing style of Mantas and Barracudas. They also had the Demiurg ships, which is why every time Tau rumors pop up in 40k, people think the Squats/Demis will appear.

The last fleet they published was Adeptus Mechanicus Explorator fleets... Basically these were Imperial ships with all sorts of advanced refits.

I totally agree with Xabre, BFG is still one of my favourite table top games, its a fantastic rule set for a very small start up price. I wrote a review of the whole system a while back, and I think most of it still stands up now...
https://www.heresy-online.net/forums/...ad.php?t=28171
The only thing I would query now would be you have to have some Ebay skillz in order to get a reasonable fleet now as GW stopped supporting it a while back.
If you ever have any questions about the game just give a shout, within my gaming group I think we have either owned or played against every race and have many battles and Campains under our belt, the best ones being a few planetary assaults that started with BFG, then Epic40k and finally 40k to finish.

I suddenly want to go fishing for a new fleet. Probably Mechanicus, because I want to build that as a 40k army... I have my Inquisition fleet right now that I need to make a Grey Knight 40k army for, too....

I have a small fleet but never played a game. Sadly, i got into it about two months before they stopped selling their line.