Nooooo, stick with Dark Eldar!
Seriously though, Tyranids are my second favourite race and I was originally going to get Tyranids, but the lack of a Codex (and thus a chance to view painted models) put me off. I got the Codex and my flatmate now plays them, albeit far too rarely.
One of the reasons that I really like Tyranids is that there are no really dud units in the Codex, a stark contrast with Eldar in particular.
I have seen two different styles of Tyranid army that have been effective. The first is a swarm army that relies on massive numbers as much as anything else and the other has a lot of the scarier monsters.
As a Dark Eldar player you should probably take a winged Hive Tyrant. It is a great modelling opportunity as well as a lethal unit. Treat it like an important Raider and all will be well. Any other choice will likely seem too slow.
All of the Heavy Support choices are excellent. Zoanthropes offer excellent power armour destruction possibilities and the Carnifex is an excellent close combat beast, as well as a good shooting platform.
Elites are good too. Lictors are fun and can either cover a lot of territory against more lightly armed foes, or provide a focussed blow against things like Marines. Warriors are all but essential. They provide Synapse and when armed with Rending Claws and Scything Talons are great in assault. Venom Cannons offer good shooting power. With various biomorphs Warriors can be anything other than Troops. Wings and Leaping legs are good, but can make them very expensive too.
The biggest choice is in the Troops section. Hormagaunts are very useful, because your opponent has very little time before they are tied up with the gaunts and can no longer see to shoot the larger creatures. Termagants are probably the least useful and most people seem to prefer Spinegaunts, but although the Spinegaunts are a couple of points cheaper they cannot hurt even a Raider(!). Their main use is that you can buy a huge number of them. Personally I would rather have gaunts with Scything Talons at one extra point, but that leaves you with the problem of deciding what to do with the masses of gun wielding gaunts that come in the plastic set. Genestealers are great. Wyches are about the only unit that can really go toe to toe with them in hand to hand. Marines fear them very much. Rippers are to my mind the most underrated unit. Before your first wave of Hormagaunts has reached combat, your enemy will be virtually guaranteed to fire everything at the larger, or middle sized creatures. If the Rippers make it to combat, it takes the rest of the game to beat them back. Make them leaping or winged for a surprise for your opponent.
Fast Attack has Gargoyles and Raveners. Gargoyles fufill a similar kind of role to Hormagaunts, but trade an extra attack for bio-plasma (better) and a gun. Generally speaking you will only need two turns of movement for either Gargoyles or Hormagaunts to reach assualt, so the mobility is nice, but not that important an advantage. Not being able to enter cover (on pain of death!) is a problem though. Raveners are quite cool, as they can put a shocking number of attacks down for such a small unit. They are also fast enough to arrive soon after the fastest Tyranids.
In my opinion the biggest decision is what proportion of your force should be dedicated to Troops and thus how much of a swarm you will have. Definitely take some Genestealers, some Warriors and probably a Tyrant, Biovores and Carnifex. Add on the swarm of gaunts and fill out the rest with whatever you feel suits your style of play the best.