The city centre of Tallinn is like a chocolate box picture, it is simply stunning. Its old city centre is walled, and within the walls almost all the building are original, with spires and quaint churches everywhere. Go up to the castle for a great view.
For a stag party Tallinn is great, as most of the bars are on a few streets around the city of the walled heart of the city, so you can easily walk from one to another. There are a couple of bars almost directly opposite on another is the centre called Nimega (Bar with a name) and Nimeta (bar with no name !) -
Nimega is a great place to go for pre clubs drink and it has its own little dance floor at the back. Ask for a guy called Adam, behind the bar and he will be able to give you some good advise on places to go to and places to avoid. Nightclub wise I would recommend Venus and Hollywood which are in the city centre and Parlament which is a taxi ride away. On lap dancing clubs I would recommend X Club - it not cheap but then again neither are the other, and this one is very good !
Accommodation wise we stayed in a hostel, on my first trip we stayed in one terrible hostel on the first night (and it really was terrible), and then moved to a pretty good hostel called Old town hostel. The second time I went we stayed in a hotel about 3 miles from the city centre and I really would not recommend it to be honest - its a real pain in the neck having to get taxi's into the city centre - a word on taxis here, everywhere we went in eastern Europe we got completely ripped of by them, they all have convenient buttons under the dashboard they press to rig their meters and I am not joking when I say we paid sometimes 8 times the price for exactly the same ride, by different firms !!
Also never, ever ask a taxi driver for recommendations for .......
...... clubs, bars, restaurants and particularly lap dancing clubs, they are all get kickbacks from where ever they take you to, and we met some guys on a stag weekend, who where taken to a strip club in a taxi over an hour away across a border, apparently the place was surrounded by barbed wire and guarded by dodgy blokes with guns !! They made an excuse that they had to go to the bank to get cash, and then had to get another taxi all the way back !
Also on being stitched up, look out for very forward girls who chat you up and ask if you want to go for a sauna. Its a scam where they get you in the place undressed and their accomplices out the back then take everything you own. I imagine getting back to your hotel without cash or clothes could be slightly embarrassing !
The one main problem I can see with Tallinn is that its nightlife centre is so small it would only take a few stag parties to totally overrun it. Therefore I think Riga is probably a better option as it has the size to absorb it without it being totally in your face. Here is more info on Tallinn stag weekends.
Overall I would give it a 8/10 as a stag weekend destination, but if it gets over run with stag parties, as I soon expect it will, then I think it may have a very short shelf life !